For soup: In saucepot over medium heat, cook onions in butter 4 min.Add remaining soup ingredients. Bring to a boil. Reduce heat; let simmer 15 min. Season with salt and pepper. Thicken with whole- wheat bread, if desired.
Meanwhile for sandwiches: Use 2 slices cheese for each. In skillet coated with cooking spray over medium-high heat, cook sandwiches in batches 5 min. or until cheese is melted and bread is golden, turning once. Cut sandwiches into cubes.