Preheat the oven to 375°F (190°C).
Clean the portobello mushrooms with a damp paper towel and remove the stems.
In a pan, heat olive oil over medium heat, then sauté garlic and bell peppers until softened.
Stir in spinach and cook until wilted, then remove from heat.
Combine cooked quinoa, sautéed vegetables, cheese, oregano, salt, and pepper in a large bowl.
Stuff each portobello mushroom cap with the quinoa mixture.
Arrange the stuffed mushrooms on a baking sheet and drizzle with a little olive oil.
Bake for 20-25 minutes until the mushrooms are tender, and the tops are golden.
Remove from the oven and let cool slightly before serving.
