Blanch okra and slice thinly. Dice tomato into small cube and cut Nagaimo into thin match sticks. Cut Shiso leaves very thinly. Keep all the vegetables cold in the fridge.
Cook dried Somen noodles according to the package. Wash well under running water. Chill in ice water if you like. Drain water when noodles are cold.
Mix Mentsuyu and cold water.
Place half of noodles in a deep dish or bowl and top with vegetables. Pour half of the sauce on top. Repeat for second serving.