Preheat oven to 350 ºF.

Cut off the tops of the tomatoes and hollow out the insides. (The pulp can be saved for use in tomato soup or sauce, or salsa.) Set tomatoes aside.

Heat oil in a saucepan over medium-high heat. Add onions, and cook until they begin to soften, about 1–2 minutes.

Add cooked vegetables, and heat through, about another 1–2 minutes.

Add quinoa, and cook gently until it smells good, about 2 minutes.

Add chicken broth, and bring to a boil. Reduce the heat and cover the pan. Cook until the quinoa has absorbed all of the liquid and is fully cooked, about 7–10 minutes

When the quinoa is cooked, remove the lid and gently fluff quinoa with a fork. Gently mix in the avocado, pepper, and parsley.

Carefully stuff about ¾ cup of quinoa into each tomato.

Place tomatoes on a baking sheet, and bake for about 15–20 minutes, or until tomatoes are hot throughout (tomatoes may be stuffed in advance and baked later).

Serve immediately.

* Unprocessed quinoa must be washed thoroughly before it is used to remove a powdery coating called saponin, which has an unpleasant and bitter taste.  Check your package for rinsing instructions.

source: National Heart Lung and Blood Institute