Remove the stem from the tomato and dice it into 1cm cubes. Slice the shiso leaves thinly.

Remove the bones from the canned miso mackerel and roughly mash it with a fork in a bowl.

Add the diced tomato and sauce ingredients (water, men tsuyu, sesame seeds) to the mackerel mixture. Mix well and chill in the refrigerator.

Boil water in a pot and cook the somen noodles according to the package instructions. Rinse under cold water and drain thoroughly.