In a medium saucepan, combine the rinsed quinoa and vegetable broth. Bring to a boil over medium-high heat. Once boiling, reduce the heat to low, cover, and let it simmer for about 15 minutes, or until the quinoa is fluffy and all the liquid has been absorbed. Remove from heat and set aside.
While the quinoa cooks, preheat your oven to 375°F (190°C).
Carefully remove the stems from the portobello mushrooms and scoop out the gills using a spoon. Brush the caps with a little olive oil and place them, gill side up, in a baking dish.
In a large skillet, heat the remaining olive oil over medium heat. Add the diced onion and red bell pepper, sautéing until softened, about 4-5 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
To the skillet, add the frozen corn, cubed zucchini, and fresh spinach. Cook until the spinach wilts and the zucchini is tender, about 3-4 minutes. Mix in the cooked quinoa, dried oregano, smoked paprika, and season with kosher salt to taste. Stir until everything is well combined.
Spoon the quinoa and vegetable mixture into each portobello cap, filling them generously. Pack the filling slightly to ensure they hold together during baking.
Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the mushrooms are tender and the tops are golden brown.
Once baked, remove the stuffed portobellos from the oven, allow them to cool for a few minutes, and serve warm.
