INGREDIENTS:
1/4 cup unsalted butter
3 pounds (about 5 medium) sweet onions, sliced
2 cloves garlic, minced
1/3 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 (3/4-inch-thick) French baguette slices
1 cup shredded Swiss cheese
1 cup shredded Gruyère cheese