Slice 4 large onions thinly
Melt 3 tablespoons butter with 1 tablespoon olive oil in a large pot over medium heat
Add the onions and cook, stirring often, until soft and deeply caramelized
Add 2 minced garlic cloves and cook for 1 minute
Stir in 1 tablespoon flour and cook for 1 minute
Pour in 6 cups beef broth or vegetable broth
Add 1 bay leaf, 1 teaspoon thyme, salt, and black pepper
Bring to a simmer and cook for 20 to 30 minutes
Remove the bay leaf
Taste and adjust seasoning
Ladle into bowls
Top with toasted bread and shredded or sliced cheese if desired
Broil until the cheese melts and turns golden