Soak radish slices in enough ume vinegar to cover, while combining remaining ingredients. Combine cabbage, carrot, celery, green onion, cucumber and lemon slices with sea salt. Massage with hands until cabbage feels "silky" and liquid squeezes from cabbage mixture. Drain ume vinegar from radishes and mix radishes into cole slaw. Place plate on top of salad and place weight on top of plate to "press" salad for 1 hour. Fluff cole slaw with fork and serve at room temperature. Serve with lemon slices as dressing.