1. Preheat the oven to 425 degrees F. Line a baking sheet with foil. Pierce the sweet potatoes and apples in several places with a sharp knife and place on the baking sheet. Bake until tender and beginning to ooze, about 30 minutes for the apples, 45 minutes to an hour for the sweet potatoes. Remove from the heat and allow to cool until you can handle them. Lower the oven to 350 degrees F and butter a 2-quart baking dish.

2. Skin the potatoes, and peel and core the apples, scraping any pulp that adheres to the skins. Cut the apples and potatoes into large pieces. Puree in a food processor fitted with the steel blade until smooth. Add the remaining ingredients and blend together well. Transfer to the baking dish. Heat in the oven until steaming, 20 to 30 minutes. Serve hot.