Shred lettuce, chop tomato, and drain and rinse beans.
Toss the lettuce and tomato together in a plastic container big enough to use as a bowl for the finished salad.
In a smaller plastic container with a tight-fitting lid, stir together the beans and salsa. Pack shredded cheese, sour cream, and tortilla chips in separate containers as well.
At lunchtime open the container of lettuce, dump the bean mixture on top, add the sour cream, sprinkle with shredded cheese, and crumble the tortilla chips on top.
Serve mango cubes on the side, and lunch is ready to go!