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.
Notes
These stuffed mushrooms can be made ahead of time by prepping the quinoa mixture and stuffing the mushrooms. Refrigerate them before baking for up to 24 hours.