Dry fry the lamb with the chopped onion, bay leaf, sliced mushrooms and diced carrots for 8-10 minutes. Add the flour and stir for a minute. Slowly blend in the stock and tomato puree. Cook, stirring, until the mixture thickens and boils. Cover and simmer gently for 25 minutes. Remove the bayleaf and place in a 1.7 litre (3 pint) ovenproof serving dish.
At the same time, cook the potatoes in boiling water for 20 minutes until tender. Drain well, mash with the butter and milk and mix well. Spread on top of the mince mixture and sprinkle over with the grated cheese.
Bake for 15-20 minutes in a pre-heated oven at 200C/400F (Gas Mark 6). Serve hot with green vegetables