Slow Cooker French Onion Soup

4 large yellow onions, thinly sliced
2 tablespoons olive oil
1 tablespoon butter
1 teaspoon salt
1/2 teaspoon sugar
1/4 cup all-purpose flour
8 cups beef broth
1/2 cup dry white wine (optional)
2 teaspoons Worcestershire sauce
1 bay leaf
1/2 teaspoon dried thyme
Salt and pepper to taste
1 baguette, sliced
2 cups shredded Gruyère cheese

1. In a large skillet, heat olive oil and butter over medium heat. Add the sliced onions, salt, and sugar. Cook, stirring occasionally, until the onions are caramelized and golden brown, about 25-30 minutes.

2. Sprinkle the flour over the onions and stir to coat. Cook for another 2 minutes.

3. Transfer the caramelized onions to the slow cooker. Add the beef broth, white wine (if using), Worcestershire sauce, bay leaf, and thyme. Stir to combine.

4. Cover and cook on low for 6-8 hours or on high for 3-4 hours.

5. Before serving, preheat your oven to broil. Place the baguette slices on a baking sheet and toast them until golden brown.

6. Ladle the soup into oven-safe bowls, place a toasted baguette slice on top of each, and sprinkle generously with Gruyère cheese.

7. Place the bowls under the broiler until the cheese is melted and bubbly. Serve hot.

For a vegetarian version, substitute the beef broth with vegetable broth. If you have picky eaters, consider using a milder cheese like mozzarella instead of Gruyère. You can also add a splash of balsamic vinegar to the soup for a touch of sweetness. For a heartier meal, add cooked chicken or beef before serving. If you prefer a thicker soup, blend a portion of the soup before adding the bread and cheese.