5 Servings - 1hr 40min.
Slice onions into ¼-inch slices. Add to a soup pot with olive oil. Begin to sauté over medium heat, stirring occasionally.
When the onions begin to get a little color (after about 30 minutes) add the butter. Continue to sauté, stirring occasionally.
Deglaze, if needed.
If the bottom of the pot begins to brown faster than the onions, add a couple tablespoons of water to dissolve the browned bits off the bottom as the onions continue to cook.
When the onions are fully caramelized (deep brown color, jammy texture), add the flour. Continue to cook and stir for about two minutes more.
Add beef broth and stir to dissolve the browned bits off the bottom of the pot.
Also add dried thyme, bay leaf, black pepper, and Worcestershire sauce. Simmer for 30 minutes.
Toast slices of bread under the broiler, just until they begin to turn a little golden brown.
Portion the soup into oven-safe bowls.
Top with bread and cheese
Top each bowl with a piece of toasted bread and some shredded Swiss or Gruyere.
Place the bowls back under the broiler for a few minutes more, or until the cheese is melted and bubbly.