Preheat the oven to 375°F (190°C).
Clean the portobello mushrooms and remove the stems.
Chop the stems finely and set aside.
In a skillet, heat olive oil over medium heat and sauté the onions until translucent.
Add garlic and the chopped mushroom stems; cook for another 2-3 minutes.
Stir in the chopped spinach and red pepper flakes; cook until spinach is wilted.
Remove the skillet from heat, then stir in quinoa and feta cheese.
Season with salt and pepper, adjusting to taste.
Stuff each portobello cap with the filling mixture.
Bake in the preheated oven for 20–25 minutes.
Remove from the oven and let cool slightly before serving.
