To make the coleslaw, use a mandoline to thinly slice the cabbage. Finely chop the herbs. Combine the cabbage, olive oil, herbs, lemon juice and salt in a bowl and leave to marinate for 5 minutes.
Toast the bread. Spread two slices of bread with mustard and mayonnaise. Place the following, in order, on one of the slices: cucumber slices, tonkatsu,