Adjust oven rack to center position and preheat oven to 375°F. Toss mushrooms with olive oil and season with salt and pepper. Transfer to a foil-lined rimmed baking sheet and spread into an even layer. Spread thyme or rosemary sprigs on top of mushrooms. Transfer to oven and roast until mushrooms release liquid, about 15 minutes. Carefully drain liquid into a separate container and reserve for another use (it’s a great vegan seasoning sauce).
Return mushrooms to oven and continue roasting until browned but still tender, about 30 minutes longer. Discard thyme or rosemary. Transfer to a bowl and toss with parsley, chives, or tarragon. Serve immediately.