Peel and cube the potatoes, then cover with salted water and bring to a boil. Cook until fork-tender, about 15-20 minutes. Drain and mash with butter, milk, salt, and pepper until smooth.

In a skillet, melt butter or olive oil over medium heat. Sauté onions and carrots for 5-6 minutes until softened.

Add minced garlic and cook for 30 seconds until fragrant.

Add ground lamb and cook, breaking it apart for 5-7 minutes until browned. Drain excess fat.

Stir in tomato paste, Worcestershire sauce, thyme, salt, and pepper. Sprinkle flour and stir for 1 minute to thicken.

Gradually pour in beef broth, stirring until incorporated. Simmer for 3-5 minutes, then add frozen peas and stir.

Spoon the lamb filling into a baking dish, layer with mashed potatoes, and smooth the surface.

Use a fork to create ridges on the potatoes. Bake at 400°F (200°C) for 20-25 minutes until golden brown.

Allow to rest for 10 minutes before serving.