Preheat oven to 425° Fahrenheit.
Combine the cheddar gruyère shredded cheese, panko, parsley, and minced garlic in a medium bowl—season with Kosher salt and black pepper. Mix to combine and set aside.
Layer one-third of the mushrooms in the bottom of a small 2-quart baking dish.
Sprinkle with one-third of the cheese mixture. Repeat the layers two more times, finishing with the cheese mixture.
Cover the baking dish and bake until mushrooms are tender, about 35-40 minutes. The mushrooms will still be firm.
Once mushrooms are cooked to your desired tenderness, remove the cover and set your oven to broil.
Broil the mushrooms until the top is crispy and golden brown.
Serve immediately.