Peel the lotus root and slice it into 7mm thick pieces. Cut V-shaped notches between the holes to create a flower shape. Soak in vinegar water for 10 minutes and drain.

Remove the mentaiko from its casing using a knife and spoon. Place it into a plastic bag and cut a small opening at the tip.

Boil enough water to cover the lotus root slices, adding 1 tbsp of vinegar. Cook the lotus root for about 3 minutes, then cool in cold water and pat dry with paper towels.

Pipe the mentaiko into the holes of the lotus root slices and smooth the surface with a spoon.