HEAT dressing in large saucepan on medium heat. Add onions; cook 10 min. or until golden brown, stirring frequently.
STIR in broth, water and thyme. Bring to boil; simmer on medium-low 5 min.
HEAT broiler. Ladle soup into 8 ovenproof bowls; top with croutons and cheese. Broil, 3 to 4 inches from heat, 2 to 3 min. or until cheese is melted.