Preheat the oven to 400° F. Wash the Brussels sprouts then trim the stems and cut the Brussels sprouts in half. Add the sprouts and mushrooms to a large bowl. Drizzle with the olive oil and season with the garlic powder, salt, and pepper. Toss to coat. Place the Brussels sprouts and mushrooms on a rimmed baking sheet and bake for 20 to 25 minutes or until the Brussels sprouts are tender when pierced with a knife.