Slice tuna into pieces about 2 x 1 x 4 (centimeters).
Mix green mustard and water, adding a few drops of water at a time to make a paste.
Neatly arrange tuna on a fancy platter, and place green mustard in a bowl.
Eat tuna sashimi by dipping fish first in mustard and then in soy sauce mixture.
Soy sauce is plain and should be poured into a small bowl.