In medium saucepan, heat oil over medium heat. Add onions, garlic and thyme; salt and pepper to taste. Reduce heat to medium-low and cook onions, stirring frequently, until golden brown (about 25 minutes).
Add broths; bring to a boil. Reduce heat; simmer 10 minutes, stirring occasionally.
Ladle soup into 4 ovenproof soup crocks or bowls; place on a baking sheet. Top each crock with croutons and cheeses. Broil until cheese is browned and bubbly.