Melt butter with olive oil in an 8-quart stock pot on medium high heat. Add onions and continually stir until tender and translucent. Do not brown the onions.

Add beef broth, sherry, and thyme. Season with salt and pepper and simmer for 30 mins.

Heat the oven broiler.

Ladle soup into oven-safe serving bowls and place one slice of bread on top of each bowl. Layer each slice of bread with a slice of provolone, 1/2 slice swiss, and 1 tbsp parmesan cheese.

Place bowls on a cookie sheet and broil in the preheated oven until the cheese bubbles and browns slightly.