Preheat oven to 400 degrees F. Coat a 9-inch baking pan with two tablespoons melted butter.
Wash potatoes. Using a mandoline or knife, slice sweet potatoes into thin rounds, about 1/8 -inch thick (microwaving potatoes for 5 minutes makes them more pliable). Place slices in prepared pan. Tuck in garlic and thyme among slices. Brush sweet potato slices with remaining two tablespoons melted butter. Season the potatoes with salt and pepper.
Bake until potatoes are tender and crisp, about 1 ¼ hours. Garnish with more thyme.
Serving suggestion: top potatoes with a dollop of sour cream.