Prepare tomatoes by slicing 1/2 from the sterm end of tomatoes and a thin sliver from the bottom if necessary to make the tomato stand upright. Scoop out the seeds and pulp, leaving a 1/4 shell. Drain pulp well and reserve the pulp for stuffing. Invert the tomatoes to drain while preparing stuffing.
In a large, non-stick sprayed skiller, melt margarine. Saure mushrooms, onion and garlic for 5 minutes. Stir in tomato pulp and croutons. Stuff shells with filling and arrange in a baking pan. Bake until shells and stuffing are thoroughly heated, usually 20 to 30 minutes in a moderate oven.