Gather all ingredients.

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

Add beef broth, sherry, and thyme. Season with salt and pepper. Let simmer for 30 minutes. Meanwhile, preheat the oven's broiler.

Ladle soup into oven-safe serving bowls and place one slice of bread on top of each (bread may be broken into pieces if you prefer). Layer each slice of bread with a slice of provolone, 1/2 slice diced Swiss and 1 tablespoon Parmesan cheese. Place bowls on a rimmed sheet pan.

Broil in the preheated oven until cheese bubbles and browns slightly, 2 to 3 minutes.

Serve hot and enjoy!