Divide the duck into joints and cook in an earthenware casserole in hot oil till evenly browned. Add the chopped onion and, when a golden brown, sprinkle with a little flour and mix thoroughly. Then add the sliced tomato, the sherry and the stock, and season with the paprika and salt. Simmer gently for about 1 hour till the duck is tender. To serve, place the pieces of duck on a hot dish, strain the sauce over them and garnish with the mushrooms and onions, previously cooked in oil or butter, and the small potatoes.