COOK onions in butter in large saucepan on medium heat 10 min. or until golden brown, stirring frequently.
ADD broth, 2 soup cans water and steak sauce. Bring to boil. Simmer on medium-low heat 5 min.
HEAT broiler. Ladle soup into 4 large ovenproof bowls; top with croutons and cheese. Broil 2 to 3 min. or until cheese is melted.