Heat the oil in a skillet/frying pan. Add spinach and saute for about a minute before adding pressed garlic. Stir constantly for a further minute or until the spinach is completely wilted. Remove from the heat.
Either clean the mushrooms with a damp cloth or peel their skin off.
In a small bowl, combine the sauteed spinach and crumbled feta (use a fork).
Fill each mushroom with about a teaspoon of the filling. Place them onto a baking tray and bake at 400°F/200°C for 20 minutes or until to your liking.
Serve warm or cold.
Notes
Extra virgin olive oil can be substituted with virgin olive oil/cooking olive oil or sunflower oil.
Avoid buying very small mushrooms as there will not be much space for the filling.
The recipe can be easily doubled or halved.
If your spinach leaves have hard stems, you can remove them.