Preheat oven to 400°.
Place all ingredients on a baking sheet and toss to combine. Spread evenly on baking sheet.
Roast for 25-30 minutes, stirring occasionally until tender and just starting to crisp.
Remove from oven and discard all but one sprig of rosemary. Transfer mushrooms to a serving platter and garnish with rosemary. Serve warm or at room temperature.