Place the green beans and mushrooms in a large bowl. In a small bowl, whisk together the olive oil, balsamic vinegar, garlic powder, salt and pepper. Pour over the green beans and mushrooms, and gently toss together until they are coated.
Preheat oven to 400 degrees F. Spread out in a single layer onto a large baking pan (do not crowd).
Bake in the preheated oven until beans are tender and mushrooms are cooked through, and all liquid on the pan has evaporated, about 20-22 minutes, flipping them half way through. Adjust seasonings to taste. Sprinkle with grated Parmesan. Serve immediately.