COVER 4 bread slices evenly with half each of the cheese and onions; top with bacon and remaining bread slices. Cut sandwiches in half. Arrange alternately with tomato slices n 13x9-inch baking dish sprayed with cooking spray
BEAT remaining ingredients with whisk until well blended; pour over ingredients in baking dish. Refrigerate 6 hours.
HEAT oven to 350ºF. Sprinkle remaining cheese and onions over ingredients in baking dish. Bake 50 min. to 1 hour or until center is set. Let stand 10 min. before serving.