Preheat oven to 350 degrees F (180 degrees F).
Heat a large skillet over medium heat and add Olive Oil (2 tablespoon). Sauté White Onion (1/2 cup) for 2-3 minutes. Add Garlic (3 clove) and continue to cook for another minute. Add Fresh Spinach (6 ounce) all at once and cook, stirring occasionally until spinach has completely wilted. Allow to cool slightly before proceeding.
To the spinach, stir in Quinoa (1 cup), Parmesan Cheese (1/2 cup), Dried Parsley (1 teaspoon), Dried Oregano (1/2 teaspoon), Dried Thyme (1/2 teaspoon), Lemon Juice (1 tablespoon), Salt (1/2 teaspoon), Ground Black Pepper (1/2 teaspoon), and Crushed Red Pepper Flakes (1 pinch). Set aside.
Rinse Portabella Mushroom Cap (4). Using a spoon, scrape out the dark fanned bit, going down to the solid surface of the mushroom. Brush the outside of each mushroom with Olive Oil (2 tablespoon).
Fill mushrooms with an approximate heaping 1/3 cup of mixture, pressing just slightly to fill in the mushroom cavity. Top with Tomato (to taste) if desired and then sprinkle each with Shredded Mozzarella Cheese (1/2 cup).
Place filled mushrooms onto a parchment lined baking sheet and bake in preheated oven for 20 minutes. Turn oven to broil and broil for 1-3 minutes until cheese is melty and golden. Remove from oven and serve immediately.