Preheat oven to 375°F (190°C).

Clean the portabella mushrooms and remove stems, creating space for the filling.

In a skillet, heat olive oil over medium heat and sauté diced onion and garlic until softened, about 3 minutes.

Add chopped spinach and tomatoes, cooking until spinach wilts.

Stir in cooked quinoa, thyme, salt, and pepper. Cook for another 2 minutes, mixing well.

Stuff each mushroom cap generously with the quinoa mixture and place them on a baking sheet.

Bake for 25-30 minutes, or until the mushrooms are tender and the top is slightly golden.

Garnish with fresh parsley and serve warm.