Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.

Clean mushrooms with a damp cloth, remove the stems, and pat dry.

In a skillet, heat olive oil over medium heat. Sauté onions and garlic until fragrant, about 2-3 minutes. Add spinach and cook until wilted. Remove from heat.

In a mixing bowl, combine cream cheese, mozzarella, Parmesan, sautéed spinach mixture, salt, and pepper. Mix until smooth and creamy.

Stuff each mushroom cap generously with the mixture. If desired, sprinkle breadcrumbs on top for a crispy finish.

Arrange stuffed mushrooms on the prepared baking sheet. Bake for 18-20 minutes, or until the cheese is bubbly and mushrooms are tender.

Let cool slightly before serving. Garnish with fresh parsley for a pop of color.