Remove the stems from the portobello mushrooms and gently scrape out the gills using a spoon. Place the mushrooms on a baking sheet.
In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the diced bell peppers, red onion, and garlic. Cook until softened, about 5 minutes.
In a large bowl, combine the cooked quinoa, sautéed vegetables, feta cheese, parsley, oregano, salt, and pepper. Mix well.
Stuff each portobello mushroom cap with the quinoa mixture, pressing gently to pack it in.
Drizzle the stuffed mushrooms with the remaining olive oil and bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is heated through.
Serve the savory stuffed portobello mushrooms hot, garnished with additional parsley if desired.