Cut off the roots of mizuna and chop into bite-sized pieces. Remove the stem of the tomato and slice thinly.

Place kiriboshi daikon in a bowl of water, rub well, and squeeze out excess water. Boil kiriboshi daikon in a pot of water over medium heat for about 5 minutes. Drain and let cool, then squeeze out excess water and cut into bite-sized pieces.

Make a slit in the mentaiko and extract the contents.

Mix the sauce ingredients (mentaiko, mayonnaise, soy sauce) in a bowl. Add kiriboshi daikon and mizuna, and mix well to combine.