Cut up the Sourdough Bread (4 slice) and toast them in the oven until they're very crispy, brown and hard (they will soften up in the soup!)
Slice the Onion (6).
Slice the Garlic (6 clove).
Melt Butter (4 tablespoon) in heavy large saucepan over medium heat. Add onions and garlic and saute until very tender and brown, about 45 minutes.
Add Dry White Wine (1/4 cup) and simmer until reduced to glaze, about 3 minutes. Stir in Low-Sodium Chicken Broth (3 cup), Beef Broth (3 cup) and Dijon Mustard (1 teaspoon). Simmer 20 minutes, then season to taste with Salt and Pepper (to taste).
Ladle soup into broilerproof bowls. Top each with slice of bread, Swiss Cheese (1 cup) and Grated Parmesan Cheese (1/2 cup). Broil in the oven until cheeses melt and bubble. Serve and enjoy!