1. Adjust rack to middle of oven. Heat oven to 425 degree F. Coat the bottom of a 1-1/2- to 2-quart glass baking dish with olive oil. Spray one side of a large sheet of aluminum foil with nonstick spray. Peel the sweet potatoes and cut into paper-thin slices. Spread them in layers in prepared pan.
2. In a large glass measuring cup, combine the garlic, chile powder, salt, thyme, chipotle chile, and pepper. Stir in milk until well blended. slowly pour the seasoned milk over the potatoes. Cover the pan tightly with prepared foil, sprayed side down, and bake 40 minutes. Remove foil and with a long-handled spoon, carefully lift and rearrange the potatoes a few times to redistribute the seasonings. Return pan to the oven and bake, uncovered, until the sweet potatoes look slightly shriveled and golden on top, about 20 minutes more. Drizzle with lemon juice and let cool 15 minutes before serving. Serve warm. Makes 6 to 8 servings.