COOK bacon in large saucepan until crisp. Remove bacon from pan with slotted spoon, reserving 2 Tbsp. drippings in pan; drain on paper towels. Add onions to reserved drippings; cover. Cook on medium heat 5 min. Uncover; cook 15 min. or until onions are golden brown, stirring frequently. Add broth, pepper and thyme; stir. Simmer 15 min. Stir in bacon.
HEAT broiler. Ladle soup into 5 ovenproof bowls; top with toast and cheeses.
BROIL, 4 inches from heat, 2 to 3 min. or until cheeses are melted and golden brown.