onion soup recipe

4 large yellow onions, thinly sliced - 4 tablespoons unsalted butter - 1 tablespoon olive oil - 1 teaspoon sugar - 1 teaspoon salt - 1/2 teaspoon freshly ground black pepper - 1/2 cup dry white wine (optional) - 6 cups beef broth - 2 sprigs fresh thyme - 2 bay leaves - 1 baguette, sliced - 2 cups grated Gruyère cheese