Our Favorite French Onion Soup

Name: Our Favorite French Onion Soup
Cuisine: French
Ingredients: 5 tablespoons unsalted butter, divided; 1 tablespoon vegetable oil; 3 pounds Vidalia onions (about 4 medium), halved lengthwise, peeled, and thinly sliced; 1 teaspoon kosher salt; ½ teaspoon freshly ground black pepper; ½ teaspoon granulated sugar; 1½ cups dry white wine; 6 cups homemade beef broth or store-bought low-sodium beef broth; 10 sprigs thyme; 2 bay leaves; 1 baguette; 1 garlic clove, cut in half lengthwise; 2 teaspoons sherry, preferably Fino or Manzanilla; 4 ounces Gruyère cheese, grated (about 1 cup); kitchen twine, six 8-ounce or four 16-ounce oven-safe ramekins or bowls (optional)