1) Heat oil in a large saucepan (with lid) over medium heat. Add onions and celery (and zucchini and bell peppers if you are using them) and cook for about 5 minutes or until soft. Add cumin and garlic and saute for about 1 minute.
2) Stir in spinach and drained tomatoes. Cook for 5 minutes or until most of the liquid has evaporated.
3) Stir in black beans, quinoa, carrots and 2 cups of water. Cover and bring mixture to a boil. Reduce heat to medium-low and simmer for 20 minutes, or until quinoa is tender.
5) Preheat oven to 350 degrees. Pour the reserved tomato juice liquid into the bottom of a baking dish. Fill each bell pepper half with the quinoa mixture and place in the baking dish. Cover with foil and bake for 1 hour.
6) Uncover and sprinkle each pepper with 1 Tbsp of the remaining cheese. Bake for an additional 15 minutes (uncovered) or until the tops of stuffed peppers are browned. Let stand for 5 minutes. Transfer stuffed peppers to a serving plate and drizzle with pan juices before serving.