01 - Preheat oven to 200°C (400°F).
02 - Boil potatoes in salted water for 15–20 minutes or until tender. Drain and mash with milk, butter, salt, and pepper. Set aside.
03 - Heat oil in a large skillet. Add onion and carrots; cook for 5 minutes.
04 - Add ground lamb and cook until browned. Stir in garlic, tomato paste, Worcestershire sauce, thyme, salt, and pepper. Cook for 2 minutes.
05 - Sprinkle in flour and stir to coat. Pour in broth, bring to a simmer, and cook until thickened (about 5–7 minutes). Stir in peas.
06 - Spread the meat mixture evenly in a baking dish. Top with mashed potatoes and smooth the surface. Sprinkle with cheddar if using.
07 - Bake for 20–25 minutes, or until top is lightly golden and filling is bubbling.
08 - Let rest 5–10 minutes before serving.