Peel sweet potatoes or yams; cut off woody portions and ends; cut into quarters. Peel, quarter, and core apples. Cook sweet potatoes, covered, in a Dutch oven in enough boiling water to cover for 15 minutes. Add apples and cook 10 to 15 minutes more or until tender. Drain.
In a large mixing bowl combine sweet potatoes and apples. Beat with an electric mixer on low speed until mashed. Add eggs; half-and-half, cream, or milk; green onions; margarine or butter; brown sugar and red pepper. Beat until well combined. Turn into a greased 2-quart souffle dish or casserole. Cover and refrigerate overnight or for up to 3 days.
To serve, bake, uncovered, in a 325 degree F oven about 1 hour or until heated through. Sprinkle with Parmesan cheese. Makes 12 servings.
Up to 3 days ahead, prepare casserole, except do not bake. Cover and chill. To serve, bake as directed.