In a large mixing bowl, combine the halved Brussels sprouts, olive oil, salt, and pepper. Toss until the Brussels sprouts are evenly coated.
Spread the Brussels sprouts onto a baking sheet in a single layer.
Roast in the preheated oven for about 20-25 minutes, or until they are golden brown and tender, flipping halfway through.
While the Brussels sprouts are roasting, prepare the glaze. In a small saucepan, combine the maple syrup, balsamic vinegar, Dijon mustard, and garlic powder over medium heat. Stir until heated through and slightly thickened, about 3-4 minutes.
Once the Brussels sprouts are done roasting, remove them from the oven and drizzle the maple glaze over them. If using, sprinkle the chopped walnuts on top.
Toss everything gently to combine, then return to the oven for an additional 5 minutes to allow the glaze to caramelize slightly.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.