Preheat your oven to 375°F (190°C).
Prepare the portobello mushrooms by cleaning and removing the stems.
In a skillet, heat 1 tablespoon of olive oil over medium heat and sauté the diced onion until translucent.
Add minced garlic and chopped spinach to the skillet, cooking until the spinach wilts.
Combine the cooked quinoa, sautéed vegetables, cheese, Italian seasoning, salt, and pepper in a bowl.
Stuff each portobello mushroom cap generously with the quinoa mixture.
Drizzle the remaining olive oil over the stuffed mushrooms.
Place the stuffed mushrooms on a baking sheet and bake for 25–30 minutes.
Remove from the oven and let rest for a few minutes before serving.