Wash and dry the tomatoes. Cut a small round from each tomato at the end opposite the stalk. Scoop out the centre with the handle of a teaspoon. Fry the onion and the mushroom in the butter or margarine until cooked. Add the ham, crumbs, parsley and sufficient tomato pulp to bind the mixture Season well. Fill the tomatoes with the mixture, piling some on top. Bake in a moderate oven (350° F., Gas 4) until the tomatoes are soft —about 20 min. Sprinkle the tops with cheese, replace the lids and serve on the fried or toasted bread. Garnish with parsley.