Start by boiling the chopped potatoes in a large pot of salted water until tender.

While the potatoes are cooking, heat a large skillet over medium heat and add the diced onion and carrots. Sauté for about 5 minutes until softened.

Add the ground lamb or beef to the skillet and cook until browned.

Stir in the minced garlic, dried thyme, salt, and pepper, cooking for another minute until fragrant.

Add the tomato paste and Worcestershire sauce, mixing well.

Pour in the beef broth and add the frozen peas, stirring to combine. Let simmer for about 10 minutes.

Once the potatoes are tender, drain them and return to the pot. Add butter, milk, salt, and mash until smooth.

Spread the meat mixture evenly in a baking dish and top with the mashed potatoes.

Bake in a preheated oven at 400°F (200°C) for about 20 minutes, or until golden brown.