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