Quinoa-stuffed Tomatoes

Quinoa (pronounced KEEN-wah) is a grain native to South America; children will have fun eating quinoa out of a hollowed-out tomato

4 medium (2½ inches) tomatoes, rinsed
1 Tbsp olive oil
2 Tbsp red onions, peeled and chopped
1 C cooked mixed vegetables—such as peppers, corn, carrots, or peas (Leftover Friendly)
1 C quinoa, rinsed*
1 C low-sodium chicken broth
½ ripe avocado, peeled and diced (see tip)
¼ tsp ground black pepper
1 Tbsp fresh parsley, rinsed, dried, and chopped (or 1 tsp dried)