Creating these delightful Stuffed Portobello Mushrooms involves just a few straightforward steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Mushrooms: Clean the Portobello mushrooms by wiping them with a damp cloth. Remove the stems and scrape out the gills gently using a spoon.
3. Sauté Vegetables: In a skillet, heat the olive oil over medium heat. Add the onion and garlic, cooking until softened and fragrant.
4. Add Spinach: Stir in the chopped spinach and cook until wilted.
5. Combine Filling: In a bowl, combine the cooked quinoa or rice, the sautéed spinach mixture, feta cheese, sun-dried tomatoes, oregano, salt, and pepper. Mix well to combine everything evenly.
6. Stuff the Mushrooms: Place the mushroom caps on a baking sheet and fill each cap generously with the stuffing mixture.
7. Bake: Place the filled mushrooms in the preheated oven and bake for 25 minutes, until they are tender and the tops are slightly golden.
8. Garnish: Remove from the oven and let cool slightly. Garnish with fresh parsley before serving.

By following these easy steps, you can whip up an extraordinary dish of Stuffed Portobello Mushrooms that are bound to impress!