4 tbsp. Unsalted Butter 3 large White Onions (2 ½ #’s, thinly sliced) 4 cup Chicken Stock/Broth 4 cup Beef Stock/Broth 2 tbsp. Balsamic Vinegar 6 slices Sourdough Bread (stale, thick slices) 2 cup Gruyere Cheese (about 6 ½ oz., grated) 2 Green Onions (chopped)
Melt the butter in a large saucepan. Add the onions and cook over moderately high heat, stirring frequently, until softened, and starting to brown, about 15-20 minutes. Add the chicken and beef stocks to the saucepan and bring to a rolling boil. Reduce the heat to low and simmer until the soup is flavorful and the onions are tender, about 1 hour. Season with salt and pepper and stir in the balsamic vinegar.
Preheat the broiler. Arrange 6 ovenproof bowls on a baking sheet. Divide the soup among the bowls, top each with a bread slice and cover the bread with the cheese. Broil for 1 to 2 minutes, until the cheese is golden and bubbling. Garnish with scallions and serve.
Make ahead: the soup can be made ahead and refrigerated for up to 1 week. Warm the soup and arrange the bread and cheese just before serving.