1. Preheat the oven to 400°F (200°C).
2. In a large skillet, cook the ground lamb or beef over medium heat until browned. Drain excess fat.
3. Add onion, carrots, and garlic to the skillet; cook until softened.
4. Stir in peas, tomato paste, beef broth, Worcestershire sauce, salt, and pepper. Simmer for about 10 minutes.
5. Meanwhile, boil the potatoes in salted water until tender. Drain and mash with butter and milk; season with salt and pepper.
6. Spread the meat mixture into a baking dish and top with mashed potatoes.
7. Bake for 25-30 minutes or until the top is golden brown.