Preheat oven to 400°F (200°C). Arrange half of the sliced sweet potatoes over the bottom of an 8-inch round (2 L) baking dish. Arrange half of the sliced onion rings over the sweet potatoes and half of the sliced apples over the onion. Repeat these layers with remaining ingredients.
In medium bowl, whisk together milk, flour, mustard, salt and pepper. Drizzle over apples. Sprinkle with cheese.
Cover with foil and bake in preheated oven for 45 minutes. Remove foil and bake until cheese is bubbly and golden and potatoes and apples are tender, approximately 15 minutes. Let stand 10 minutes before serving.
*Variety Tip: For best results, use Ontario Empire, Golden Delicious, Spartan, Cortland or Northern Spy apples.