In a large mixing bowl place the tuna. Incorporate the remaining ingredients and mix thoroughly. Refrigerate tuna until needed.
In a bowl combine all ingredients. Incorporate evenly and reserve until needed.
Place all ingredients in a bain-marie. Wrap tightly with plastic and place in a double boiler and allow to steep for 3 hours. Allow sauce to cool then strain through a china cap. Reserve until needed.
To plate place a mound of tuna tartare in the middle of the plate and surround with cucumber relish. Drizzle ponzu sauce around. Serve immediately.
Recipe courtesy Charlie Palmer, Aureole, NYC and Vegas