Preheat your oven to 375°F (190°C).

Carefully clean your large fresh mushrooms and remove their stems. You can chop the stems finely and add them to your stuffing mixture later.

In a bowl, combine melted butter, mozzarella cheese, minced clams, chopped green onion, beaten egg, minced garlic, bread crumbs, and dried oregano. Mix everything thoroughly until well combined.

Generously fill each mushroom cap with your prepared stuffing mixture using a spoon or your hands.

Arrange the stuffed mushrooms on a baking sheet and drizzle them with additional melted butter if desired. Sprinkle grated parmesan cheese on top before placing them in the oven for about 25 minutes or until they are cooked through and golden brown.