Once beans are soft, strain beans into a large colander nested inside a bowl to retain the liquid. Set onion aside and return cooked beans to slow cooker. Add butter and mash with potato masher. Stir reserved liquid back into beans by 1/2 cup measurements until desired consistency is reached. Season generously with salt and pepper to taste. Do not discard reserved liquid. The starch in the beans will continue to develop as the beans sit, so additional liquid can be stirred in. Save the liquid...
