Heat pan and saute leeks. Do not caramelize, keep them white. Deglaze with mirin and reduce all liquid. Toss in butter and check for seasoning. Lay salmon paillard to cover 2/3 of nori sheet. Lay cooled leeks on salmon and roll. Save 1/4 of the leeks for plate up. In a stainless steel bowl, whisk soda water into rice flour until a paint consistency is achieved. Heat oil to 375 degrees. Dip roll into tempura and fry until golden brown. The salmon should end up medium rare at most!