In a large skillet,saute'onions in butter for 5 minutes.
Add croutons and saute 3 minutes
Place tomato slices in a bowl.Sprinkle with salt,pepper and basil;toss gently.
Spoon half of reserved onion mixture into greased 13 by 9inch baking dish.Arrange half of tomato slices over onion mixture;sprinkle with half of cheese.
Bake, uncovered,at 350 degrees for 35 to 40 minutes until hot and bubbly.