Wash, wipe, and stalk the tomatoes. Melt the butter in a frying-pan, put in the onion, and fry it a pale brown; then add the crumbs, parsley, brown sauce, and half the cheese. Stir the mixture over the fire until it is hot and thoroughly mixed. Season it carefully.
With a sharp-pointed knife, cut out a round piece from the stalk end of each tomato. Scoop out some of the soft part, making a cavity in which to put the filling. This requires great care, otherwise the knife may go through the tomato, and it will burst when put in the oven. Fill each tomato with some of the prepared mixture, heaping it up slightly on the top. Mix the rest of the cheese with the browned crumbs, and sprinkle a little on each tomato. Put them on a baking-tin or in a fireproof dish, and cook them in a moderate oven from five to eight minutes, or until they are just tender. Be careful they are not overcooked, as they will lose their shape.