Preheat the oven to 400°F (200°C).

Place the cubed potatoes in a large pot of cold salted water. Bring to a boil and cook for 12-15 minutes until fork-tender.

While potatoes boil, heat a large oven-proof skillet over medium-high heat. Add the ground lamb and cook until browned, breaking it apart with a spatula. Drain excess fat.

Add the diced onion and carrots to the skillet with the meat. Sauté for 5-7 minutes until the vegetables have softened.

Stir in the minced garlic, tomato paste, thyme, and rosemary. Cook for 2 minutes until fragrant.

Pour in the beef broth and Worcestershire sauce. Bring to a simmer and cook for 8-10 minutes, allowing the liquid to reduce and thicken into a savory gravy.

Stir in the frozen peas and season the meat mixture with salt and pepper. Remove from heat.

Drain the cooked potatoes and return them to the pot. Mash with butter and heavy cream until smooth, then stir in the shredded cheddar cheese.

Spread the mashed potatoes in an even layer over the meat mixture in the skillet. Use a fork to create decorative ridges on the surface for extra browning.

Bake for 20 minutes, or until the potato topping is lightly browned and the filling is bubbling around the edges.