Cucumber Salsa: Combine all ingredients and marinate overnight in the refrigerator.
Wasabi Vinaigrette: Put the first 4 ingredients in a blender or food processor. Slowly add oil as you blend to form a thick paste. Add salt and pepper as desired.
Tuna Preparation: Coat the tuna with a generous amount of ground black pepper (the entire surface area). In a hot saute pan, sear each side of the tuna ever so slightly. The fish should remain raw. Chill.
Slice thinly and garnish with salsa and vinaigrette.