French onion soup - Au Pain Doré

Ingredients

• 1/4 cup unsalted butter
• 3 lbs (about 6) finely sliced yellow onions
• 2 garlic cloves, finely chopped
• 1/3 cup white wine
• 6 cups beef broth
• 1 tablespoon chopped fresh thyme
• 2 bay leaves
• 2 teaspoons white wine vinegar
• Salt and pepper
• 1 Au Pain Doré Poolish baguette , sliced
• 1 cup Swiss cheese
• 1 cup Gruyère cheese