Wash mushrooms and remove stems, pat dry. Save stems for another recipe. In a mixing bowl, place clams, onions, garlic salt, minced garlic, soft butter and oregano. Mix thoroughly. Add Italian bread crumbs, egg, and clam juice and blend. Stir cheeses in clam stuffing and mix well.
Place clam mixture inside mushroom cavity and slightly mound clam mixture. You should be able to stuff between 8 and 12 mushrooms depending on the size of mushrooms.
Place mushrooms in lightly oiled baking dish. Pour melted butter over mushrooms. Cover and place in a preheated oven at 350 degrees for about 35 and 40 minutes. Remove cover, sprinkle freshly grated mozzarella cheese on top of the mushrooms and pop them back in the oven just until the cheese melts slightly. Garnish with freshly diced parsley.