Instructions:

1. In a large stock pot, melt butter and oil over medium heat.
2. Add the onions and sit well to coat in the butter and oil. Cook over medium heat for 30-40 minutes, until browning, stirring occasionally. Continue to cook for an additional 1 hour on medium or medium low, stirring occasionally. (You want the onions to get extremely soft and tender and a deep, rich brown color but not burn.)
3. Add the sherry and cook for 1-2 minutes to allow some of it to evaporate.
4. Add the broth, thyme, Worcestershire sauce, salt and black pepper. Stir well. Add the cubed bread and stir to incorporate.
5. Bring to a boil then reduce heat, cover and simmer on medium low for 30 minutes.
6. For serving: Carefully transfer the soup to individual ramekins or individual tureens or small oven-proof bowls. Sprinkle with 2 tablespoons of shredded Swiss cheese. Add the slice of French bread on top, then top the bread with Gruyere cheese.
7. Broil on high heat, about 3-4 inches from the heat source, for 3-4 minutes, until the cheese is melted, being careful not to let it burn. Serve immediately.
8. (Alternatively, you can place the bread slices on a foil lined baking sheet, top with Gruyere cheese and broil for 2-3 minutes, until melted. Sprinkle the soup bowl with Swiss cheese and serve with the broiled bread slice on top.)