Clean and skin the kidneys, remove the core and slice the kidneys in half. Fry the onions in the fat until soft. Add the kidneys and sausages and fry for 3 minutes turning once. Remove and place in 3-pint casserole dish. Stir the flour into the hot fat and mix well with a wooden spoon. Add a little at a time the mixture of stock, beer, sherry and tomato puree. Season to taste and cook over a medium heat for a few minutes stirring constantly. Sprinkle the sliced mushrooms over the kidneys and pour on the sauce Cover and cook at gas mark 4, 350 F (180 C) for l and half hours.