Melt butter with olive oil in an 8 quart sauce pan on medium heat. Add onions and stir until tender and translucent.
Add beef broth and thyme. Season with salt and pepper. Simmer for 30 minutes.
Heat the oven broiler.
Pour soup into oven safe bowls and place one slice of bread on top of each (I like to break slice into pieces). Layer bread with 1/2 slice of Swiss, 1 slice of provolone, and 1 tbsp of Parmesan cheese. Place bowls on cookie sheet and broil in oven until cheese browns slightly.