Cheesy Mushroom Gratin

Preheat oven to 450 degrees F. Toss mushrooms, oil, pepper and salt together in a large bowl. Place the mushrooms in a single layer on a wire rack and set the rack on a rimmed baking sheet. Roast until well browned and tender, about 25 minutes. Remove from oven and let cool for 5 minutes.

Reduce oven temperature to 400 degrees F. Combine mozzarella, Gruyère, thyme and 2 teaspoons parsley in a medium bowl. Place 6 mushrooms in a single layer in a shallow 7-by-11-inch baking dish; top with one-third of the cheese mixture. Repeat the mushroom and cheese layers 2 times. Combine panko and the remaining parsley in a small bowl; sprinkle evenly over the top. Bake until golden brown and bubbly, about 15 minutes.