01 - Set the oven temperature to 400°F.
02 - In a large bowl, combine Brussels sprouts and sliced red onion. Add olive oil, balsamic vinegar, sea salt, black pepper, and thyme, tossing until evenly coated.
03 - Spread the mixture evenly on a large baking sheet lined with parchment paper.
04 - Roast for 20 minutes, stirring once halfway through to ensure even caramelization.
05 - Add the chopped chestnuts to the baking sheet, gently toss with the sprouts and onions, then roast an additional 10 minutes until Brussels sprouts are golden and chestnuts warmed through.
06 - Transfer the contents to a serving dish and serve immediately while hot.