Preheat oven to 375°F and line a baking sheet with parchment paper.

Sauté spinach, garlic, and onion in a drizzle of olive oil over medium heat until spinach wilts and onions turn translucent, about 3 minutes. Remove from heat and cool slightly.

Mix cooled vegetables with cream cheese, mozzarella, and Parmesan in a bowl. Stir until smooth, creamy texture forms and ingredients are evenly combined.

Brush each cleaned mushroom cap with olive oil, ensuring underside and edges are coated.

Stuff each cap with about 1 tablespoon of filling, pressing gently to mound mixture slightly above the rim.

Bake for 18–20 minutes at 375°F until mushrooms are tender and cheese is melted and golden at the edges.

Optional: Sprinkle fresh parsley over hot mushrooms.