After washing the mushrooms, carefully remove the stems and chop them, setting them aside for another use.
Preheat the oven to 350 degrees Fahrenheit and spray a baking dish with a non-stick cooking spray.
In a mixing bowl, stir together all the filling ingredients, including the minced clams, garlic, green onion, butter and oregano, egg, breadcrumbs, and cheese.
Place the stuffing mixture inside each mushroom cavity and slightly mound it for a generous fill.
Arrange the stuffed mushrooms in the lightly oiled baking dish, ensuring they are not touching.
Cover the baking dish with foil and bake for 20-30 minutes until the mushrooms are tender.
Remove the foil and sprinkle each mushroom with the grated mozzarella cheese, then bake for an additional few minutes until the cheese is golden and bubbly.