For the tomatillo salsa: Rinse the tomatillos with cool water. Cut into small pieces and add to the bowl of a food processor along with the onion and garlic and process until very smooth. Add the cilantro, lime juice and jalapeno and process again until well combined. Place the mixture in a small saucepan. Add the cumin and some salt and pepper and bring to a boil over medium heat. Cook, stirring occasionally, until most of the liquid has evaporated and the consistency is chunky, about 15 minutes. Cool before using. The salsa will stay fresh in a covered container in the refrigerator for up to 1 week.
For the stuffed tomatoes: Preheat the oven to 450 degrees F. Bring the filtered water to a boil in a saucepan. Add the quinoa and cook over moderate heat until the water is absorbed, 12 to 15 minutes. Remove the quinoa from the heat and fluff it with a fork.
Combine the black beans, vinegar, pepper and 1/2 teaspoon salt in a medium to large mixing bowl. Add the quinoa, corn, cilantro, bell pepper, celery, green onion, pumpkin seeds, jalapeno, lemon juice, oil, garlic, cumin and cayenne. Mix well to combine.
Slice off the upper part of each tomato and remove the tops. Gently scoop out the pulp and seeds with a small spoon and discard. Sprinkle with salt and fill each tomato with a generous amount of the quinoa mixture. Sprinkle the top of each tomato with some nutritional yeast. Place the tomatoes in a lightly oiled 13- by 9-inch baking dish and bake until slightly softened and fork-tender, about 20 minutes. Remove from the oven and serve immediately with the tomatillo salsa.