Preheat the oven to 400F and line a baking sheet with parchment paper.
Remove and discard the mushroom stems. Dry clean the mushrooms with the help of a brush or a clean paper towel to remove any dirt.
Chop the tomatoes and mince the garlic. Next, boil a small pot full of water. Add thawed spinach in a colander and set it over the sink.
As soon as the water boils, hold the colander firmly with a towel and carefully pour the boiling water over the spinach so it immediately wilts the leaves.
Once the leaves are wilted, immediately run under cold running water to refresh the leaves. Squeeze the spinach in your hands to remove the excess water.
Transfer the squeezed spinach to the chopping board and chop finely.
In a bowl mix together spinach, sun-dried tomatoes, cream cheese, half the parmesan cheese, mozzarella cheese, garlic, salt, and pepper.
In another bowl prepare the topping; combine, remaining half of the parmesan cheese, panko breadcrumbs, oregano, and olive oil. Stir until well combined.
For assembling, spray the mushrooms with cooking oil spray and place the caps onto the prepared baking sheet. Spoon the cream cheese filling into the mushroom caps.
Add a layer of topping on top and bake for 20-25 minutes until mushrooms are cooked through and the top is golden and crispy.
Serve warm and enjoy!