Mushrooms get tender in the oven before they're smashed and broiled, creating crispy edges. An herbaceous, bright gremolata completes these tasty mushrooms.
Position racks in upper and lower thirds of oven; preheat to 475°F.
Wipe mushrooms clean and trim stems flush with caps. Score a 1/4-inch X across the cap of each mushroom. Toss the mushrooms, oil, 1/4 teaspoon salt and pepper together in a large bowl until the oil is absorbed. Arrange the mushrooms on a large rimmed baking sheet. Roast on the lower rack until browned and very soft, about 20 minutes.
Meanwhile, chop parsley. Add lemon zest, garlic and the remaining 1/4 teaspoon salt to the parsley on the cutting board and continue chopping until finely chopped.
When the mushrooms are done, remove them from the oven and turn the broiler to high. Turn the mushrooms stem-side up on the pan and use the bottom of a mason jar or other sturdy glass to flatten each mushroom. Broil on the upper rack until crisp on the bottom and edges, 3 to 4 minutes. Serve topped with the gremolata.