Preheat oven to 350 degrees F. Lightly coat a 2- to 2-1/2-quart baking dish with cooking spray; set aside. Using a sharp knife or a mandoline, cut sweet potatoes horizontally into 1/4-inch-thick slices. Set aside.
For sauce, in a medium saucepan melt butter over medium heat. Stir in flour; cook and stir for 1 minute. Stir in half-and-half. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more.
Reduce heat to low. Add 3/4 cup of the cheese, stirring until cheese is melted. Stir in thyme, salt, and pepper.
Place potato slices in the prepared baking dish. Pour sauce over potatoes; sprinkle with the remaining 1/4 cup cheese.
Bake, covered, about 50 minutes or until potatoes are tender. Bake, uncovered, about 20 minutes more or until edges are brown. Let stand for 15 minutes before serving.
From the Test Kitchen
Dont like thyme? Replace it with rosemary. Love bacon? Scatter 1/4 cup crumbled, crisp-cooked bacon on top before serving.
