Preheat your oven to 425°F (220°C).
Rinse the green beans under cold water and trim the ends. Clean the mushrooms with a damp cloth and slice them into thick pieces.
In a mixing bowl, combine the green beans, sliced mushrooms, olive oil, garlic powder, salt, and pepper. Toss until evenly coated.
Line your baking sheet with parchment paper. Spread the seasoned green beans and mushrooms in a single layer.
Roast in the oven for about 15-20 minutes, tossing halfway through for even cooking.
Remove from the oven, drizzle with balsamic vinegar, and sprinkle grated parmesan over the top. Return to the oven for an additional 2-3 minutes.
Let cool for a minute, then serve warm.