Frying bacon in the skillet, then tossing sprouts and red onion in the drippings before roasting means you'll only dirty one pan making this elegant side dish.
In a 12-inch cast-iron or other heavy oven-going skillet, cook and stir bacon over medium heat until bacon is browned and crisp. Use a slotted spoon to remove bacon to paper towels to drain. Add crushed red pepper to drippings in skillet; cook and stir for 1 minute or until fragrant.
Add Brussels sprouts, onion, thyme, and salt to skillet. Stir to coat with drippings. Transfer skillet to a 400°F. oven and roast, uncovered, for 20 to 25 minutes or until just tender and browned, stirring once.
Transfer to a serving dish and sprinkle with reserved bacon.