Wash and dry the romaine lettuce. Drain and rinse the black beans in a colander. If using fresh corn, cook and cool it before adding. Dice the red bell pepper and avocado, halve the cherry tomatoes, and finely chop the red onion and cilantro. Place all prepared vegetables and beans into a large mixing bowl.
In a small bowl or jar, whisk together the lime juice, olive oil, cumin, and a pinch each of salt and pepper. Taste and adjust salt if needed.