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 and lightly browned This can take a while, so be patient.
Add beef broth, sherry, and thyme. Season with salt and pepper. Let simmer for 30 minutes.
Meanwhile, lightly toast the bread. Then preheat the broiler.
Ladle soup into oven-safe serving bowls and place one slice of lightly toasted bread on top of each. Layer each slice of bread with the grated cheese (as much as you want).
Place bowls on a cookie sheet and broil in the preheated oven until cheese bubbles and browns slightly, 2 to 3 minutes. Serve hot!
