Quickly rinse and then soak palm fronds in water for 1 hour. Peel and slice onion in crescents and steam for 60 seconds. Slice cucumber in 1/4" thick half moons. Drain palm fronds and slice in 2" lengths. Mix with onion and cucumber and season with umeboshi plum vinegar and let marinate for 4-12 hours. (Note: for quicker method, pressure cook palm fronds for 5 minutes in a little water, marinate with other ingredients for 1 hour). Garnish with nasturtiums or sesame seeds, and serve.