Cut the tops off of the tomatoes. Very gently use a knife to cut around the inside of the tomato, to loosen it. Use a spoon to gently scoop out the inside of the tomatoes, being careful not to pierce the outside wall.

When the tomato is cleaned out inside, place it cut side down on a kitchen towel or paper towels for 5 minutes or so to drain some of the excess liquid out of the inside of the tomatoes.

Put 2 cups of water in a medium sized pot. Rinse your quinoa in a strainer under running water for 30 seconds or so, then add it to the pot. Bring the water to a boil, then lower the temperature to low and cook for 15 to 20 minutes, until all of the water has been absorbed, similar to cooking rice.

In the meantime, add 1 teaspoon of cooking oil to a frying pan and heat to medium. Add the chopped onion and saute until it is beginning to get soft. Add the garlic and spinach and cook for another few minutes.

When the quinoa is cooked fluff it with a fork then add it to the onion and spinach mixture. Mix together well and add salt and pepper to taste, as well as the Parmesan cheese. I also add a bit of the inside of the tomato that you scooped out, chopped, seeds removed. It adds a bit of moisture to the mixture.

Preheat the oven to 350ºF.

Stuff the tomatoes with the quinoa and spinach filling. Top each tomato with shredded mozzarella cheese if desired. Bake for 20 to 30 minutes, until the tomatoes are cooked and the filling is hot.