Cook Udon according to the package. Wash in running cold water, strain well, and keep refrigerated.

Slice green onions thinly. Tear Nori into small pieces. Mix Mentsuyu and cold water in a bowl and set aside.

Place cold noodles in a serving bowl, top with Tenkasu, green onions, and Nori, and pour the Mentsuyu mixture over the noodles. Sprinkle with sesame seeds.