Ingredients
3 pounds yellow onions, peeled, sliced, and cut into quarter-moons
2 tablespoons unsalted butter, melted
2 tablespoons olive oil
1 teaspoon salt, plus more to taste
Freshly ground black pepper
2 tablespoons balsamic vinegar
10 cups beef broth
3 tablespoons brandy, optional

To Serve
4 to 6 baguette slices, toasted, for each bowl
1 1/3 to 2 cups grated Gruyere cheese (1/3 cup per bowl)