Thinly slice the potatoes, stack them slightly offset, and cut into matchsticks. Soak in water for 3–4 minutes, then drain. Boil the potatoes for 1–2 minutes, then cool in cold water and drain.

Cut off the root ends of the kaiware sprouts and slice them in half lengthwise.

Make a slit in the mentaiko and extract the contents.

In a bowl, mix the mentaiko with the seasonings (mayonnaise, shiro dashi, salt, and pepper). Add the potatoes and mix well.