Preheat your oven to 400°F (200°C).

In a large skillet over medium heat, brown the ground lamb or beef until fully cooked. Drain excess fat if needed.

Add the diced onion and carrots to the skillet and cook for about 5 minutes or until the vegetables are tender.

Stir in the tomato paste, Worcestershire sauce, and broth. Bring to a simmer, and cook for an additional 5 minutes. Add salt and pepper to taste.

Fold in the frozen peas and remove the pan from heat.

Transfer the meat and vegetable mixture into a baking dish, and spread the mashed potatoes evenly on top.

Bake for 20-25 minutes or until the top is golden brown and crispy.

Let it cool for a few minutes, garnish with fresh thyme if desired, then serve.