Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the cooked quinoa, chopped spinach, cherry tomatoes, mozzarella cheese, Parmesan cheese, minced garlic, oregano, olive oil, salt, and pepper. Mix well until all ingredients are evenly combined.
Place the portobello mushrooms, gill side up, on a baking sheet lined with parchment paper.
Generously spoon the quinoa mixture into each portobello mushroom cap, pressing down slightly to pack the filling.
Bake in the preheated oven for 25-30 minutes, or until the mushrooms are tender and the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes. Garnish with fresh basil leaves if desired before serving.