Preheat the oven to 180 degrees Celsius.
Make an incision in each chestnut and roast them in the oven for 20 minutes. Remove them from the heat and wrap in a tea towel. Peel each chestnut while it's still hot and cut into slices approx 5 mm thick.
Steam the brussel sprouts until they are tender, approx 10 minutes. While the sprouts are cooking, heat the olive oil and cook the garlic and chestnuts for approx 2 - 3 minutes until the garlic is golden.
Toss the garlic and chestnuts through the sprouts and season.
Note: always pierce chestnuts before roasting them otherwise they will explode in the oven and continue to explode when you pull them out. At best it'll make a terrible mess, at worst you could burn badly.