You can cook the rice and beans in advance, or for an even easier short cut, use frozen or ready-to-eat rice and canned beans. The onions, peppers, and corn can be chopped and/or cooked in advance as well. The fresh veggie toppings (such as cherry tomatoes, lettuce, avocado, and cilantro), are best added right before serving, or no longer than the night before so they don't get wilted or soggy.