Trim fat from the cutlets and coat them with flour. Melt the butter and fry the cutlets and sliced onion until golden. Put into a casserole with the mushrooms, stock, salt and pepper and rosemary. Peel the potatoes and cut in quarter-of-an-inch slices. Arrange the potatoes on top of the meat, cover and bake at 300°F, 150°C, gas mark 2 for one and a half hours. Take off the lid and continue baking for thirty minutes.