Preheat oven to 325 degrees. Peel potatoes; cut into 1/4 inch slices. Place potatoes in a large pot of boiling water. Cook for 5-10 minutes or until just tender. Drain well and set aside.
Meanwhile, fry bacon in a large skillet over medium-high heat until crisp. Drain on a paper towel, crumble and set aside.
Add chopped onion to bacon drippings and cook until tender. Stir in flour over low heat and cook to a paste. Add salt and pepper. Add milk and cook until mixture thickens slightly.
Arrange half of the sweet potatoes in the bottom of an 11×7 inch baking dish. Sprinkle on half of the crumbled bacon. Pour on half of the milk mixture. Arrange remaining potatoes, sprinkle with remaining bacon and pour the rest of the milk mixture over the top. Sprinkle with cheese. Bake for 20 minutes or until potatoes are tender. If desired, brown cheese for 1-2 minutes under the broiler.