Melt the butter in a large non-stick pan over a medium heat. Add the carrots, onion and mushrooms and cook for 3 to 4 minutes. Increase the heat, add the mince and fry for a further 3 to 4 minutes, until the lamb has browned.
Stir in the stock, wine, tomato purée, Worcestershire sauce and some seasoning. Gently bubble for 15 to 20 minutes, until the mince is completely browned and the sauce has thickened. Pour into the base of a pie dish and set aside.
Meanwhile, peel the potatoes and cut into chunks. Place in a pan of water and cook for 25 to 30 minutes, until softened, then drain. Mash the butter into the potatoes and season with a little salt and pepper.
Spoon the mash onto the mince and cook in the oven for approximately 30 minutes, until bubbling and golden brown. Serve immediately.