Cool, peel, and slice sweet potatoes. Pare and slice apples. Place the maple syrup in a saucepan and add butter and apples. Cook gently until apples are tender. Layer the sweet potatoes and apples in a buttered casserole. Pour sauce-pan juices over them. Just warm in oven or place in refrigerator to use later*.
*To make these extra delicious, run the hot casserole under the broiler long enough to brown the top.