Preheat oven to 350 degrees F. In a 4-quart Dutch oven cook sweet potato slices in lightly salted boiling water for 5 minutes; drain and set aside. Grease a 2-quart oval or rectangular baking dish; set aside.
For sauce, in a small saucepan cook onion in hot butter about 5 minutes or until tender. Stir in flour and salt. Add milk all at once; stir in chipotle pepper. Cook and stir over medium heat until thickened and bubbly. Remove saucepan from heat; stir in cheese.
Place half of the sliced potatoes in prepared baking dish. Cover with half of the sauce. Repeat layers.
Bake, covered, for 25 minutes. Uncover and bake about 15 minutes more or until potatoes are tender. Let stand for 10 minutes before serving. Makes 8 to 10 servings.