Gratin is the ultimate comfort food side dish, and this decadent herb and Gruyère root vegetable gratin elevates the classic to a holiday masterpiece. Instead of using only white potatoes, we layer thinly sliced sweet potato, parsnip, and Yukon Gold potatoes, which creates a stunning visual appeal and a complex balance of earthy, sweet, and savory flavors. The entire dish is bound together by a rich, savory cream sauce infused with garlic and covered in a bubbling, golden Gruyère crust.
This recipe is designed to be a showstopper. While it takes some time to slice and bake, the prep work can be done entirely ahead of time, allowing you to simply slide the covered dish into the oven when your guests arrive. It’s rich, creamy, and guaranteed to be the most requested side dish at any dinner party or holiday gathering.
Why You’ll Love This Recipe
- Stunning Presentation: The alternating colors of sweet potato, parsnip, and white potato create a beautiful, elegant layered effect.
- Perfectly Creamy Texture: The thin slices of potato and root vegetables cook tenderly in the heavy cream, naturally thickening the sauce without turning grainy or mushy.
- Complex Flavor Blend: The nutty, savory Gruyère cheese and aromatic fresh thyme beautifully complement the natural sweetness of the root vegetables.
- Ideal Make-Ahead: You can assemble the entire gratin up to 24 hours in advance, then bake it just before serving.
- A Hearty Side: This dish is substantial enough to hold its own alongside any rich protein.
Ingredients
For the Root Vegetables
- 1 lb (450 g) Yukon Gold or Russet potatoes, peeled
- 1 lb (450 g) sweet potatoes, peeled
- 1/2 lb (225 g) parsnips (about 3 large), peeled
For the Cream Sauce
- 2 cups (480 ml) heavy cream
- 1/2 cup (120 ml) whole milk
- 2 cloves garlic, minced
- 1 tbsp chopped fresh thyme leaves
- 1/2 tsp ground nutmeg (optional, but enhances savory flavor)
- 1 1/2 tsp salt
- 1/2 tsp black pepper
For the Cheese Topping
- 1 cup (115 g) Gruyère cheese, freshly shredded
- 1/2 cup (50 g) Parmesan cheese, freshly grated
Equipment
- 9×13 inch (23×33 cm) baking dish (or a 3-quart oval dish)
- Mandoline slicer (highly recommended) or sharp knife
- Small saucepan
- Large bowl
Step-by-Step Instructions
1. Prepare Vegetables and Infuse Cream
- Preheat and Prep Dish: Preheat your oven to 400°F (200°C). Generously grease the baking dish with butter or cooking spray.
- Slice Thinly (Crucial!): Using a mandoline or a very sharp knife, thinly slice all the peeled root vegetables (potatoes, sweet potatoes, parsnips) into uniform slices, about 1/8-inch to 1/4-inch (3 mm) thick. Uniformity is key for even baking.
- Infuse Cream: In a small saucepan, combine the heavy cream, milk, minced garlic, thyme, nutmeg, salt, and pepper. Heat over medium-low heat until the cream just begins to bubble around the edges. Do not boil. Remove from heat and set aside to cool slightly.
2. Layer and Soak the Gratin
- Layer Base: Arrange a single, overlapping layer of the sliced vegetables in the prepared baking dish. Aim for an alternating pattern (e.g., Potato, Sweet Potato, Parsnip) for a nice visual effect.
- Add Cheese and Repeat: Sprinkle a small amount of the Gruyère and Parmesan cheese over the first layer. Continue layering the vegetables, cheese, and herbs until all the slices are used. Make the top layer as neat as possible.
- Pour Liquid: Gently and evenly pour the infused cream mixture over the entire dish, ensuring the liquid seeps down between all the slices.
3. Bake to Perfection
- Covered Bake: Cover the baking dish tightly with aluminum foil. Bake for 50–60 minutes, or until the vegetables are tender when pierced with a fork.
- Uncover and Brown: Remove the foil. Sprinkle the remaining Gruyère and Parmesan cheese over the top. Bake, uncovered, for an additional 10–15 minutes, or until the top is bubbly, deeply golden brown, and the sauce is set.
- Rest and Serve: Allow the gratin to cool and set for at least 15 minutes before slicing and serving. This resting time prevents the creamy sauce from running out.
Substitutions and Variations
- Lighter Sauce: For a less rich dish, substitute the heavy cream with whole milk or a blend of milk and chicken broth. You may need to add 1 tablespoon of flour to the cold milk before heating to help it thicken.
- Vegetable Swap: Substitute the parsnips with carrots, turnips, or celery root. If using beets, pre-roast and add them near the end to prevent their color from bleeding into the entire dish.
- Cheese Blend: Substitute the Gruyère/Parmesan with a blend of sharp white cheddar and Havarti for a different, yet equally gooey, texture.
- Vegan Option: Substitute the heavy cream with full-fat canned coconut milk and the cheeses with a plant-based cheese alternative or nutritional yeast.
Pro Tips and Common Mistakes to Avoid
- Uniform Slices are Key: Because the vegetables are all baked together raw, they must be sliced to the same thickness (ideally 1/8 to 1/4 inch). A mandoline slicer makes this job quick and perfect.
- Infuse the Cream: Heating the cream with the garlic and herbs allows those savory flavors to fully infuse the liquid before it goes into the oven, ensuring the entire gratin is seasoned.
- Freshly Grate the Cheese: Pre-shredded cheeses contain anti-caking agents that interfere with melting and can make your sauce grainy. Always grate your own cheese for a smooth, cohesive gratin.
- Allow Resting Time: Resting the gratin for 15 minutes after baking allows the hot liquid to thicken fully and the layers to set, resulting in clean, firm slices.
Storage, Make-Ahead, and Reheating
- Make-Ahead: You can fully assemble the gratin (through step 3) up to 24 hours in advance. Cover it tightly with foil and refrigerate. Add 15–20 minutes to the covered baking time to account for the cold temperature.
- Storage (Leftovers): Store leftovers in an airtight container in the refrigerator for up to 4 days.
- Freezing: Gratin freezes well. Cover the cooled, baked dish tightly with foil and freeze for up to 3 months. Thaw overnight in the refrigerator.
- Reheating: Reheat slices in a 350°F (175°C) oven for 15–20 minutes until bubbling hot and the crust is crisp.
Serving Suggestions
This rich gratin is the perfect side dish for any holiday or special occasion roast. It pairs beautifully with roasted prime rib, roasted chicken, or baked ham. A simple green salad provides a welcome contrast to the richness.
Approximate Nutrition
- Yields: 8 servings
- Serving Size: 1/8 of the casserole
- Calories: 410 kcal
- Protein: 14 g
- Fat: 30 g
- Carbohydrates: 22 g
Note: These are approximations and can vary based on the specific root vegetable mix and the fat content of the cream used.
FAQs
Q: How is this different from scalloped potatoes?
A: Scalloped potatoes are typically only potatoes and often made with a simpler milk sauce. Gratin is distinguished by its use of cheese (like Gruyère or Parmesan) layered throughout, and often a crunchy topping or browned crust.
Q: Why do I cover the gratin with foil at the start?
A: Covering the gratin traps steam, which ensures the raw, dense root vegetables have enough moisture and time to soften completely without the top cheese layer burning before the center is cooked.
Q: Can I use pre-shredded cheese?
A: We strongly advise against pre-shredded cheese. The anti-caking agents will prevent the sauce from becoming smooth and creamy, leading to a grainy or separated sauce.
Q: Can I skip the fresh herbs?
A: You can use dried herbs instead (use 1/3 the amount—1 teaspoon dried thyme and 1/2 teaspoon dried rosemary). However, fresh herbs, especially thyme, provide a brighter, more aromatic flavor that truly elevates the dish.
Q: Can I omit the white potato to make it lower carb?
A: Yes. Substitute the white potato with an equal amount of celery root (celeriac) or rutabaga, which are lower in starch. This will alter the flavor slightly but will still bake into a delicious gratin.
Herb and Gruyère Root Vegetable Gratin
Equipment
- 9×13-inch baking dish
- Mandoline slicer or sharp knife
- Small saucepan
- Large bowl
- Aluminum foil
Ingredients
- 1 lb Yukon Gold or Russet potatoes, peeled
- 1 lb sweet potatoes, peeled
- 0.5 lb parsnips, peeled
- 2 cups heavy cream
- 0.5 cup whole milk
- 2 cloves garlic, minced
- 1 tbsp chopped fresh thyme leaves
- 0.5 tsp ground nutmeg (optional)
- 1.5 tsp salt
- 0.5 tsp black pepper
- 1 cup Gruyère cheese, shredded
- 0.5 cup Parmesan cheese, grated
- butter or cooking spray, for greasing dish
Instructions
- Preheat oven to 400°F (200°C). Grease a 9×13-inch baking dish with butter or cooking spray.
- Using a mandoline or sharp knife, slice all peeled root vegetables into uniform 1/8–1/4 inch (3 mm) thick slices.
- In a small saucepan, heat heavy cream, milk, minced garlic, thyme, nutmeg, salt, and pepper over medium-low until just bubbling around edges. Remove from heat; do not boil.
- Arrange a layer of vegetables in the prepared dish, alternating colors. Sprinkle some Gruyère and Parmesan. Repeat layering until all ingredients are used.
- Pour the infused cream mixture evenly over the layers so the liquid seeps down between slices.
- Cover tightly with foil and bake for 50–60 minutes, or until vegetables are tender when pierced with a fork.
- Remove foil, sprinkle remaining Gruyère and Parmesan on top, and bake uncovered for 10–15 minutes until golden and bubbling.
- Let cool and set for 15 minutes before slicing and serving.
