In a large pot, combine the rinsed quinoa and vegetable broth. Bring it to a boil, then reduce to a simmer, cover, and let it cook for about 15 minutes or until the quinoa is fluffy and the liquid is absorbed. Remove from heat and let it sit covered for 5 minutes.
While the quinoa is cooking, preheat your oven to 375°F (190°C).
Using a spoon, gently remove the stems and gills from the portobello mushrooms. Brush each cap with a bit of olive oil and sprinkle with salt. Place them in a baking dish, open side up, and set aside.
In a skillet, heat the extra virgin olive oil over medium heat. Add the diced onion and red bell pepper. Sauté for about 5 minutes until they begin to soften. Next, add the minced garlic, zucchini, and corn. Cook for an additional 3-4 minutes, stirring frequently.
Once the vegetables are tender, stir in the fresh spinach, cooked quinoa, dried oregano, and smoked paprika. Season with kosher salt to taste. Mix everything together until well combined and the spinach wilts.
Generously fill each portobello cap with the quinoa and vegetable mixture. Press down gently to compact the filling and ensure each mushroom is well-stuffed.
Cover the baking dish with foil and bake in the preheated oven for 25-30 minutes. Remove the foil during the last 5 minutes of baking to slightly brown the tops.
Once done, remove the stuffed mushrooms from the oven and let them cool for a few minutes. Serve warm.
