French Onion Soup Ingredients 3 pounds yellow onions, cut into 1/8-inch pieces 3 tablespoons unsalted butter 1/2 teaspoon table salt, plus more as needed 1 pinch sugar 8 cups water , plus more as needed 1 tablespoon all-purpose flour 1 sprig fresh thyme 1/4 cup dry white wine 1 baguette , crusty 8 ounces Gruyère