Place beans, water, chopped onion and salt in slow cooker.  Cook on high for 4-6 hours; or when beans have cooked through.

Drain beans, reserving liquid.  Place beans into a bowl and mix them with a hand mixer, or masher. Add reserved water when needed to help mash beans until you have reached your desired consistency.

Warm tortillas, top each one with bean mixture, sprinkle shredded lettuce on top, then roll up and wrap in plastic wrap.  Allow rolls to set in the refrigerator for at least an hour, remove and cut into bite sized pieces. Top each sushi with shredded cheese, diced tomato and pepper.