In a frying pan or large saucepan, pour in a bit of oil, then saute the onion and garlic until the onion is semi-transparent.Add in the lamb mince and stir until completely browned.Add the rosemary leaves, beef stock, mushroom sauce, and carrots, and let simmer for 20 minutes.In a casserole dish, spread a layer of mashed potato on the bottom.Pour in the lamb mince mixture, then thickly layer the rest of the mashed potato on top.Place dabs of butter on top, and place in the oven for 30-45 minutes.Remove and serve.