Mushrooms add an earthy flavor to this dish. Feel free to use any variety of mushrooms, substituting them for the baby portabella mushrooms suggested in this recipe. Button mushrooms are readily available and offer a mild flavor, while wild mushrooms, such as enoki or shiitake, offer a richer flavor. In a taste panel, we thought this was reminiscent of the flavors of French Onion Soup, with the added meatiness of the mushrooms, in a delicious casserole.