In a large saucepan with enough water to cover sweet potatoes, cook over medium heat until tender but still firm about 25-30 minutes. Remove from heat. Let cool before slicing. Wash and core apples. Slice sweet potatoes and apples 1/4" thick.
Arrange a layer of potatoes/apples in a buttered baking dish. Sprinkle with brown sugar, grated orange peel, salt, and butter. Repeat, then cover and bake in a 375 degree oven for 40 minutes, or until tender.