Prepare an ice bath. Bring a medium pot of water to a boil; add brussels sprouts, and cook for 2 minutes. Using a slotted spoon, transfer brussels sprouts to ice bath and let cool.
Drain brussels sprouts, discarding ice bath. On work surface, halve brussels sprouts. In a large, shallow saucepan, heat duck fat until it just begins to smoke; add Brussels sprouts cut-side down. Pan-roast until golden brown, and toss while hot with garlic and thyme. Remove thyme; season with salt and pepper. Serve immediately.