Peel the marrows and cut into halves lengthwise or cut into rings, about 2 in. thick. Remove the seeds with a tablespoon and steam the marrow until almost soft. Drain carefully. Fry the chopped mushrooms and onion in the butter until cooked—about 10 min. Add all the other ingredients and bind with the egg and enough stock to make a soft stuffing. Season well. Stuff the marrow halves or rings of marrow with filling. Sprinkle browned crumbs on top, then a little melted butter. Bake in a fairly hot oven (375° F., Gas 5). Serve with brown or tomato sauce.