Preheat the oven to 375°F (190°C).
Clean the portobello mushrooms with a damp cloth.
Remove the stems and gills from the mushrooms.
In a skillet, heat olive oil over medium heat and sauté garlic until fragrant.
Add chopped spinach and diced tomatoes; cook until wilted.
Stir in cooked quinoa, mixing well to combine.
Season the mixture with salt and pepper.
Spoon the quinoa mixture into each portobello cap.
Top each stuffed mushroom with shredded cheese.
Bake for 20-25 minutes until the mushrooms are tender and cheese is bubbly.