Preheat oven to 350°. Line 2 rimmed baking sheets with parchment paper. In a large bowl, toss sweet potatoes, olive oil, salt, and pepper to coat. Arrange in a single layer on prepared baking sheets and roast until browned on top, about 30 minutes. Flip slices over and roast until browned, another 20 to 30 minutes.
Meanwhile, cook pancetta in a medium frying pan over medium heat, stirring often, until browned, 8 to 10 minutes.
Cut brie in lengths to fit sweet potato slices. Set a piece of brie on each sweet potato slice and sprinkle pancetta and pistachios on top. Return to oven to melt cheese slightly, 4 to 5 minutes. Transfer sweet potatoes to a platter, then sprinkle with thyme.