Preheat oven to 400 degrees.
Slice brussels sprouts in half and place on a baking sheet. Drizzle on the olive oil, sprinkle on the salt, then toss to coat the brussels sprouts and the pan with oil.
Spread out the brussels sprouts evenly on the pan with the cut side down, moving smaller sprouts to the middle to prevent burning.
Roast in the oven for 15 - 18 minutes or until tender.
While the sprouts are roasting, in a large bowl combine together the balsamic reduction, mustard, and garlic powder. Once brussels sprouts are cooked through, add them to the bowl and toss to coat. Serve immediately.