Tomato and Caviar

4 slices rye bread 2 tbs. butter 4 leaves Boston lettuce 3 med. tomatoes, thinly sliced 1/2 c. chopped onion 4 tsp. black caviar (lumpfish) 1 lemon, quartered Butter bread. Arrange a lettuce leaf on each slice. Arrange thin tomato slices to cover bread. Combine onion and caviar; place on top of tomatoes. Serve with lemon quarters. Makes 4 sandwiches.