Preheat your oven to 400°F (200°C).

In a large skillet over medium heat, cook the ground lamb until browned, breaking it apart with a spoon. Drain excess fat if necessary.

Add the chopped onion, diced carrots, and minced garlic to the skillet. Sauté for about 5 minutes until the vegetables are softened.

Stir in the tomato paste, beef broth, Worcestershire sauce, dried thyme, salt, and pepper. Bring the mixture to a simmer and cook for 10 minutes to allow flavors to meld.

Stir in the frozen peas and remove the skillet from heat.

Transfer the meat mixture to a baking dish and spread it evenly.

Top with the prepared mashed potatoes, spreading them to cover the meat completely.

Dot the top with butter and sprinkle with shredded cheddar cheese if using.

Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the filling is bubbling.