Bring 2⅓ cups of dashi, 1 tablespoon of mirin, 1 teaspoon of sugar, 1½ tablespoons of soy sauce, and ⅛ teaspoon of kosher salt to a simmer. Adjust the salt to taste.
Cook 2 portions of frozen udon noodles in boiling water following the package instructions. Drain.
Divide the noodles between bowls, pour over the hot broth and top with thinly sliced scallion.
