How to Make It
In a bowl, gently stir the mayonnaise with the mentaiko, sesame oil and 1/2 cup of water.
In a large, deep skillet, combine the noodles with the chicken broth and bring to a boil. Cover and cook over moderately high heat until the noodles are softened and the broth is nearly absorbed, about 3 minutes. Add the grapeseed oil, then stir in the mushrooms and scallions and cook, stirring occasionally, until the broth is completely absorbed and the scallions are softened, about 2 minutes. Stir in the mentaiko sauce and cook, tossing, just until the noodles are evenly coated. Garnish with the shiso and nori and serve.
Look for Kewpie mayonnaise (which is made with rice vinegar and very popular in Japan), mentaiko and fresh green shiso leaves at Asian markets.