Prepare the filling: Heat a large pan with oil and brown the minced lamb. Add chopped onion, garlic, carrots, and cook until soft.
Season: Add peas, rosemary, thyme, salt, pepper, Worcestershire sauce, and beef stock. Simmer for 10-15 minutes until thickened.
Boil potatoes: While the filling simmers, boil the potatoes in salted water for 10-15 minutes or until tender.
Mash the potatoes: Drain the boiled potatoes and mash with butter, milk (or cream), salt, and pepper until smooth.
Assemble: Preheat the oven to 200°C (fan 180°C/gas mark 6). Spread the lamb filling evenly in a baking dish, then top with the mashed potatoes. Use a fork to create ridges on top for a crispy finish.
Bake: Place in the oven and bake for 25-30 minutes or until the top is golden brown and crispy.
Serve hot and enjoy!