3 pounds yellow onions, peeled, cut in half and thinly sliced

2 tablespoons unsalted butter, melted

2 tablespoons olive oil

2 ½ teaspoons kosher salt

½ teaspoon freshly ground black pepper, or more to taste

2 sprigs fresh thyme

¼ cup balsamic vinegar

8 cups beef broth, ([2] 32oz. cartons) 

⅓ cup brandy

4 to 6 baguette slices, toasted (1 slice per bowl)

1 ⅓ to 2 cups grated Gruyere cheese (⅓ cup per bowl)

Directions:

Put the onion slices in a 6 Quart Slow Cooker, along with the butter, olive oil, salt, and black pepper.

Cover the 6 Quart Slo Cooker and cook on low overnight or for 10-12 hours. 

The onions should be dark golden brown and soft.

Add the beef broth, balsamic vinegar, and fresh thyme.

Cover and continue cooking on low for 6 to 8 hours.

Season to taste with additional salt and pepper.

Stir in the brandy.

Preheat the oven to 350 degrees Fahrenheit (180 degrees Celcius).

In even portions, ladle the soup into oven-safe bowls. 

Place the bowls on a baking sheet to catch any spills while in the oven.

Top each bowl with a baguette slice and the shredded Gruyere cheese.

Bake for 20 to 30 minutes, until the cheese is completely melted.

Turn the oven to broil and brown the tops for 2 to 3 minutes, or until the cheese is bubbling and browned. 

Remove from the oven and let cool for a few minutes. 

Enjoy!