Trim the root ends of enoki mushrooms, cut them in half, and separate the root clusters. Slice the shiitake mushroom cap and stem thinly. Trim the root ends of shimeji mushrooms and separate them.

In a pot, combine mentsuyu and water, and bring to a boil over medium heat. Add enoki, shiitake, and shimeji mushrooms. Cover and simmer on low heat for 5 minutes.

Uncover the pot and add the potato starch mixture (1 tbsp starch + 2 tbsp water). Stir continuously over medium heat until the soup thickens. Slowly pour in the beaten egg and cook until set.

Place the frozen udon noodles on a microwave-safe plate, cover loosely with plastic wrap, and heat according to the package instructions.