Preheat the oven to 350°F.
Lightly oil a small baking dish.
Gently wash and stem mushrooms; pat dry (Save stems for another recipe).
Combine clams, onions, garlic salt, minced garlic, butter and oregano in a bowl and mix well.
Add Italian bread crumbs, egg, clam juice, mixing well.
Add Parmesan, Romano, and Mozzarella cheeses to the clam stuffing and mix well.
Place approximately 1 ½ teaspoons of the clam stuffing mixture inside mushroom cavity and slightly mound (You should be able to stuff between 8- 12 mushrooms depending on the size of mushrooms).
Place the stuffed mushrooms in slightly oiled baking dish and pour ¼ cup melted butter over the top of the mushrooms.
Cover and place in oven for about 35- 40 minutes.
Remove cover; sprinkle the ¼ cup freshly grated mozzarella cheese on top and pop back in oven just so the cheese melts slightly.
Garnish with freshly diced parsley.
Serve hot.
