Set the oven at 400 degrees. Butter an 8-inch square baking dish.
In a large saucepan fitted with a steamer insert, combine the squash and potatoes. Bring to a boil, cover the pan, and steam over high heat for 15 minutes or until the vegetables are tender when pierced with a skewer.
Using a food mill or ricer set over a bowl, work the vegetables to form a puree. Or mash them with a potato masher until they are coarsely pureed. Add the zucchini, ricotta, nutmeg, parsley, 1/4 cup of the Fontina or blue cheese, and plenty of salt and pepper.
Transfer the mixture to the baking dish. Smooth the top. Add the remaining 1/4 cup cheese, butter, and a sprinkle of nutmeg.
Bake for 30 minutes or until the top is golden. Let the dish rest for 5 minutes before serving.