Preheat the oven to 350°F (175°C).

Remove the stems from the mushrooms and finely chop them.

Heat the olive oil in a skillet over medium heat. Add the chopped mushroom stems, onion, and garlic. Cook until the onion is translucent and the mushrooms have released their moisture, about 5 minutes.

Add the spinach to the skillet and cook until wilted, about 2 minutes. Remove from heat and let cool slightly.

In a bowl, combine the cooked spinach mixture, feta cheese, Parmesan cheese, breadcrumbs, dried oregano, salt, and pepper.

Stuff each mushroom cap with the spinach and cheese mixture, mounding it slightly.

Place the stuffed mushrooms on a baking sheet and bake for 20-25 minutes, or until the mushrooms are tender and the filling is golden brown.

Serve hot and enjoy!