Cut the potatoes into small bite-sized pieces and soak them in water. Drain and place them in a microwave-safe container with 1 tbsp water. Cover loosely with plastic wrap and microwave at 600W for 7 minutes. Drain and mash the potatoes until smooth while still hot.

Make a slit in the mentaiko and extract the contents.

Mix the mashed potatoes with the remaining filling ingredients (mentaiko, vinegar, mayonnaise, salt, and pepper). Divide into 6 portions and let cool slightly.

Place a spring roll wrapper in a diamond shape. Lay a shiso leaf (underside facing up) vertically in the center. Add 1/6 of the filling on top of the shiso leaf. Fold the sides of the wrapper inward and roll into a stick shape. Seal the end with the flour-water mixture. Repeat for all 6 rolls.