01 - Set the oven temperature to 400°F (200°C).
02 - In a large bowl, toss Brussels sprouts, red onion, and garlic with olive oil, rosemary, salt, and pepper until evenly coated.
03 - Spread the mixture uniformly on a parchment-lined baking tray.
04 - Roast for 20 minutes, stirring once halfway through to ensure even cooking.
05 - Incorporate chopped chestnuts into the tray, gently toss, and continue roasting for an additional 8 to 10 minutes until Brussels sprouts are golden and chestnuts are heated through.
06 - Remove from oven and, if desired, sprinkle with lemon zest and fresh parsley before serving.