Preheat oven to 425°F. Line a baking sheet with parchment paper.
Toss together potatoes, mushrooms and onions in a large bowl; drizzle with oil. Add salt, pepper, and 1 Tbsp rosemary; toss well to coat.
Spread vegetables in a single layer in prepared pan; roast, stirring once, until potatoes are tender and browned, about 30-35 minutes. Toss with parsley and remaining 1 Tbsp rosemary; serve.