Apple and sweet potato gratin
3 lbs sweet potatoes, peeled and sliced in 1/4-inch thick round
1 onion, thinly sliced
4 apples, peeled, cored and sliced in 1/4-inch thick rounds
1 cup milk
1 tbsp all-purpose flour
1/2 tsp dry mustard
1/2 tsp each salt and pepper
1 cup shredded Gruyere cheese
Arrange half of the sliced sweet potatoes over the bottom of an eight-inch round or square baking dish. Arrange half of the sliced onions over the sweet potatoes and half of the sliced apples over the onions. Repeat these layers with remaining ingredients.
Heat oven to 400 F.
In medium bowl, whisk together milk, flour, mustard, salt and pepper. Drizzle over apples. Sprinkle with cheese.
Cover with foil and bake in oven for 45 minutes. Remove foil and bake until cheese is bubbly and golden and potatoes and apples are tender, about 15 minutes. Let stand 10 minutes before serving.
