Wash quinoa under cold water several times. Drain and cook quinoa in boiling water for 12 to 15 minutes. Cook quinoa in salted water for 10 minutes. Drain in a sieve and let quinoa cool and dry.
Combine red kidney beans with balsamic vinegar, salt and pepper to taste.
Combine quinoa with beans, red pepper, ají (chili), cilantro, corn, celery, garlic, and green onion. Season to taste with salt, pepper, lemon juice, cumin and olive oil. Set aside while preparing the tomatoes.
Cut upper part of tomatoes horizontally. Remove pulp and seeds with a spoon, carefully. Sprinkle salt and invert on a rack for 15 minutes.
Stuff tomatoes with quinoa and bean mixture. Place stuffed tomatoes over salad on a plate. Garnish with lettuce leaves.