Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
Clean mushrooms with a damp paper towel; set aside.
Heat olive oil in a pan over medium heat. Sauté minced garlic for 1 minute until fragrant.
Add chopped spinach to the pan and cook for 2–3 minutes, until wilted and moisture has evaporated.
Remove from heat and let the spinach mixture cool slightly.
In a bowl, mix cream cheese, Parmesan, mozzarella, breadcrumbs, cooled spinach mixture, salt, and pepper until smooth.
Spoon the mixture into mushroom caps, mounding slightly.
Place on prepared baking sheet and bake for 15–20 minutes, until mushrooms are tender and cheese is golden.
Garnish with chopped parsley before serving, if desired.