1. Melt butter with olive oil in pot over medium heat. Add onions and continually stir until tender and translucent.
  2. Add beef broth, sherry, and thyme. Simmer 30 minutes.
  3. Ladle soup into oven safe bowls and place one slice of bread on top of each. Layer bread with slices of provolone and a 1/2 slice Swiss.
  4. Broil in the preheated oven until cheese browns slightly.