Start by preheating your oven to 375°F (190°C). Lightly grease a baking sheet with olive oil or use parchment paper for easy cleanup. Gently clean the mushroom caps using a damp paper towel to remove any dirt. Twist off the stems and discard or save for another recipe.
In a medium bowl, combine the thawed and well-drained spinach with softened cream cheese, sour cream, Parmesan, mozzarella, and minced garlic. Add a pinch of salt and pepper. Mix everything together until it’s well blended and creamy. The mixture should be thick but easy to spoon.
Use a spoon or mini scoop to carefully stuff each mushroom cap with the spinach dip filling. Don’t overfill, but make sure the tops are slightly rounded. Arrange the stuffed mushrooms on the prepared baking sheet, spaced slightly apart. Bake in the oven for 15–20 minutes until the mushrooms are tender and the tops are golden and bubbly.
Once baked, remove the mushrooms from the oven and let them cool for 2–3 minutes. Transfer to a serving plate while still warm. These are best served hot, but they’re also delicious at room temperature. Garnish with a sprinkle of extra Parmesan or chopped parsley if you like.