Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or lightly grease it.
Gently clean the mushrooms with a damp paper towel and remove the stems. Place the mushroom caps, cavity-side up, on the prepared baking sheet. Finely chop the mushroom stems and set aside.
Heat the olive oil in a skillet over medium heat. Add the chopped onion and sauté for 2-3 minutes until softened. Stir in the minced garlic and chopped mushroom stems, cooking for another 2 minutes.
Add the chopped spinach to the skillet and cook for 2-3 minutes until wilted. Remove from heat and let cool slightly. In a mixing bowl, combine the spinach mixture with the feta cheese, cream cheese, Parmesan cheese, oregano, salt, and black pepper. Stir until well blended.
Spoon the filling into the mushroom caps, pressing gently to pack it in. Arrange the stuffed mushrooms evenly on the baking sheet. Bake for 18-20 minutes, until the mushrooms are tender and the tops are golden.
Remove from the oven and let cool slightly. Garnish with fresh parsley if desired. Serve your spinach and feta stuffed mushrooms warm as a delicious appetizer or side!