Preheat the oven to 350 degrees F. Toss the potatoes, mushrooms, olive oil, thyme and rosemary in a large bowl. Season with some salt and pepper and toss again.
Spread the vegetables on a large baking sheet and dot with the butter. Roast until golden brown and fork tender, about 20 minutes. Remove the thyme sprigs and serve hot.
Recipe courtesy of Guy Fieri