In a small microwave-proof bowl, combine the beans and 1/2 cup of the salsa. Microwave on high until hot, 1 to 2 minutes. Tear the lettuce into bite-size pieces. Divide the taco shells among 4 plates (2 per plate). Divide the bean mixture among the taco shells and top with the lettuce, Cheddar, and the remaining 1/4 cup of salsa.