Ingredients
- ¼ cup unsalted butter
- 3 pounds medium sweet onions, sliced
- 2 cloves garlic, minced
- ⅓ cup dry white wine
- 6 cups beef stock
- 4 sprigs fresh thyme
- 2 bay leaves
- 2 teaspoons white wine vinegar
- Kosher salt and freshly ground black pepper, to taste
- 12 French baguette slices
- 1 cup shredded Swiss cheese
- 1 cup shredded Gruyère cheese