Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a large bowl, combine the cooked quinoa, chopped spinach, cherry tomatoes, feta cheese, red onion, garlic, olive oil, oregano, salt, black pepper, and red pepper flakes. Mix well until all ingredients are evenly incorporated.
Place the cleaned portobello mushrooms on the prepared baking sheet, gill side up. Spoon the quinoa mixture evenly into each mushroom cap, pressing down gently to pack it in.
Bake in the preheated oven for 25-30 minutes, or until the mushrooms are tender and the tops are slightly golden.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.