Preheat the oven to 350 degrees F. Lightly grease a baking dish large enough to hold the mushrooms in a single layer. Clean the mushrooms. Remove the stems, keeping the caps intact. Place the mushrooms in the baking dish stem-side up In a bowl, mix together the clams, onion, garlic salt, garlic, cooled melted butter, and oregano. Mix well. Add the breadcrumbs, egg, clam juice, Parmesan, Romano, and Mozzarella cheese. Mix well. Lightly mound the clam stuffing into each mushroom cap. Drizzle the warm melted butter over each mushroom. Cover the baking dish with foil. Place the baking dish in the oven and bake at 350 degrees F for 35-40 minutes or until the mushrooms are tender. Remove the foil from the baking dish. Sprinkle each mushroom with Mozzarella cheese. Return to the oven, uncovered, and bake until the cheese is melted. Garnish the mushrooms with the parsley and serve hot.