Wipe mushrooms clean with a damp paper towel — do not rinse. Remove stems and set aside. Scrape out any remaining stem bits from caps. Brush caps lightly with olive oil, season with salt and pepper, and place cavity-side up in a baking dish.
Finely chop reserved mushroom stems. Melt butter in a skillet over medium heat. Add garlic and cook 1 minute. Add chopped stems and cook 3–4 minutes until moisture has evaporated and the pan looks dry. Remove from heat and cool slightly.
Combine the cooked mushroom mixture with drained clams, breadcrumbs, cream cheese, parmesan, parsley, onion powder, black pepper, red pepper flakes, and reserved clam juice. Stir until the mixture holds together when pressed. Taste and adjust seasoning. Add more breadcrumbs if too wet, more clam juice if too dry.
Spoon filling generously into each mushroom cap, mounding slightly above the rim.
Preheat oven to 400°F. Scatter mozzarella and parmesan over each filled cap. Bake for 20–22 minutes until mushrooms are tender and cheese is melted and golden. Broil for 2 minutes at the end if the cheese needs more color.
Let mushrooms rest 3–5 minutes before serving. Spoon any pan juices over the tops. Finish with fresh parsley and serve warm.
