Preheat oven to 350 degrees F (175 degrees C). Line a 9x13 inch baking dish with a sheet of aluminum foil, and lightly spray with nonstick cooking spray. Clean the mushrooms carefully with a damp paper towel or a soft mushroom brush, and place into the prepared pan upside down.
Place the clams, 1/2 cup of melted butter, onion, Parmesan cheese, bread crumbs, and garlic in a large mixing bowl. Season with parsley, Italian seasoning, and pepper; mix well. Fill the mushroom caps with the clam mixture, and round over the tops. Sprinkle with the shredded mozzarella cheese, and drizzle with the remaining 1 cup of butter.
Bake in the preheated oven until the mushrooms are tender, and the mozzarella cheese has lightly browned, 25-30 minutes.