Whisk the mayonnaise and mustard together in a large bowl. Pulse the ham and pickles in a food processor until finely chopped but not pureed. Add the ham mixture, pickle juice, celery, onions and celery leaves to the mayonnaise mixture, and stir. Taste; if the salad seems dry (it should be spreadable), add more mayonnaise and/or pickling liquid. Serve as is or on sandwiches.