Preheat oven to 350°F (175°C) and lightly grease a 9x13-inch ceramic or glass baking dish.

Place a large skillet over medium-high heat and add the ground sausage. Cook until browned and fully cooked, breaking it into small crumbles with a spatula for approximately 7 minutes.

Drain excess grease from the skillet and spread the cooked sausage crumbles in an even layer across the bottom of the prepared baking dish.

Distribute the chopped baby spinach evenly over the sausage layer.

In a large mixing bowl, whisk together the eggs, heavy whipping cream, salt, black pepper, and garlic powder until the mixture is homogeneous.

Incorporate 1 cup of the shredded cheddar cheese into the egg mixture.

Pour the egg and cheese mixture over the sausage and spinach in the baking dish, using a fork to ensure the liquid reaches the bottom of the pan.

Top the casserole with the remaining 1 cup of shredded cheddar cheese and the sliced green onions.

Bake in the center rack of the oven for 30 to 35 minutes, or until the center is firm to the touch and the edges are slightly golden.

Remove from the oven and allow the casserole to set for 5 minutes at room temperature before slicing into 8 rectangular portions.