Onion Mixture:

• 4 cups sweet onion, thinly sliced (from 4 medium/large onions)
• 2 cups shallots, thinly sliced (from about 4 shallots)
• 1 teaspoon kosher salt
• 1 2/3 tablespoons extra virgin olive oil

Soup Base:

• 2 tablespoons all-purpose flour (or gluten-free flour)
• 1/4 cup light white wine
• 1/4 cup sherry vinegar
• 4 fresh thyme sprigs
• 2 dried bay leaves
• 8 cups chicken bone broth
• 1 pound boneless skinless chicken thighs or breasts

For Serving:

• 6 slices baguette or sourdough bread, toasted
• 12 thin slices Gruyere or Swiss cheese