Heat olive oil in medium saucepan. Add the onion and saute until tender, about 5 minutes.
Add the garlic, salt, and pepper, and saute another two minutes longer. Add the quinoa, tomato pulp, and water and stir to combine.
Taste a little and add more salt or pepper if desired. Place the cored tomatoes into a casserole dish. Spoon the quinoa mixture inside the tomatoes and place to tomato tops back on.