In an enameled cast-iron casserole, combine the onions and butter with 1 1/2 cups of water and bring to a boil. Season with salt and pepper, cover and cook over moderate heat until the onions are softened, 10 minutes. Uncover and cook over moderately low heat, stirring, until the onions are soft and just beginning to caramelize, 40 minutes longer. Add a few tablespoons of water as the pan dries out. The <em>fonduta</em> will be thick and creamy.