Wash sweet potatoes. Cook unpeeled sweet potatoes, covered, in small amount of boiling salted water until tender, about 20 to 30 minutes. Cool. Preheat oven to 350 F. Peel and slice potatoes into 1/4-inch thick rounds; set aside. Melt butter in medium saucepan. Add onion, cook until tender. Remove from heat; stir in flour, salt, basil, mustard and pepper. Cook over low heat until bubbly, 2 to 3 minutes. Remove from heat; stir in milk all at once. Cook over medium heat, stirring constantly, until mixture comes to a boil. Boil and stir one minute. Remove from heat. Stir in 3/4 cup of the cheese and the ham. Pour half of mixture into greased 2-quart casserole. Arrange half of sweet potatoes on top. Repeat layers. Bake for 25 minutes. Sprinkle with remaining cheese and bake 3 to 4 minutes longer, or until cheese has melted. Makes 6 servings.