To make the rice, add salt and rice to a pot of boiling water. Return to a boil, then reduce heat, cover, and simmer until water is absorbed and rice is tender, 15-18 minutes. Uncover and fluff with a fork, then toss in lime zest, lime juice, cilantro, and additional salt to taste.
To assemble, arrange rice and lettuce in the bottom of a serving bowl and top with the diced tomatoes, diced avocados, corn, black beans, and red onions.
