Preheat oven to 400 degrees. Slice each sweet potato in half lengthwise, and slice each half crosswise into 1-inch-thick half-moons.
Melt butter in a large cast-iron or other ovenproof skillet over medium heat. Add sweet potatoes; toss to coat evenly. Add maple syrup; toss potatoes to coat, and bring to a boil.
Transfer skillet to oven; cook, stirring occasionally, until potatoes are golden, well glazed, and tender when pierced with a paring knife, about 20 minutes. Remove from oven, and season with salt and pepper. Serve warm, garnished with thyme.