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.
While the chestnuts are cooling, toss the Brussels sprouts and garlic through the oil in a roasting pan and season with cracked pepper. Place in the oven to roast for 20 minutes or until tender.
Remove from the oven and mix the chestnuts through the sprouts. Sprinkle with lemon juice, season with a pinch of salt, and serve.
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.