Caramelize onions: Add butter and onions to a large pot over medium heat. Season with a little salt and pepper and cook until softened. Add sugar and continue to gently stir until onions become golden brown and caramelized, about 25 minutes. Careful to not let them burn! Maintain a medium to medium-low heat.

Add the flour and cook for 3 more minutes. Add beef stock, wine, thyme, celery, and bay leaves. Cook partially covered for 30 minutes.

Preheat oven to 350 degrees F.

Toast bread: Arrange bread on a baking sheet and brush with melted butter on both sides. Bake for 15 minutes, flipping to the other side half way through. Remove from oven and rub a cut clove of garlic onto each piece of bread.

Broil: Turn oven to High broil. Remove bay leaves and the piece of celery from the soup and ladle soup into 4 oven proof bowls. Place the toasted bread on top of each bowl. Depending on the size of your bowls you may need two slices of bread. Sprinkle the tops generously with both types of cheese. Broil until the cheese is browned and bubbling.