Preheat your oven to 400°F (200°C).

Peel and chop the Yukon Gold potatoes into even chunks. Boil in salted water for 15-20 minutes until fork-tender. Drain and mash with butter, milk, salt, and pepper.

In a skillet, heat oil over medium heat. Sauté onion and carrots for 5 minutes, then add garlic and cook for another minute.

Add ground lamb, browning for 5-7 minutes until no longer pink.

Stir in tomato paste and Worcestershire sauce, then sprinkle in flour, mixing well.

Pour in stock and simmer for 10 minutes until thickened. Add peas and season with salt and pepper.

Spread the meat filling in a baking dish and top with mashed potatoes, creating ridges with a fork.

Bake at 400°F (200°C) for 20-25 minutes until golden brown on top.

Let cool for a few minutes before serving.