Preheat the oven to 350°F (180°C).

Remove the stems from the mushrooms and finely chop them.

In a large bowl, combine the chopped mushroom stems, spinach, feta cheese, breadcrumbs, garlic, olive oil, salt, and pepper.

Stuff the mushroom caps with the mixture and place them on a baking sheet lined with parchment paper.

Bake in the preheated oven for 15-20 minutes, until the mushrooms are tender and the filling is golden brown.

Serve hot as a party appetizer or a side dish.