Remove the thin skin from the mentaiko and extract the roe. Crack the egg into a bowl and beat it. Boil water in a pot and cook the soba noodles according to package instructions. Drain the noodles.

Add the seasoning ingredients (mirin, soy sauce, dashi powder, and water) to a pot and heat over low heat. Once it starts to boil, add the mentaiko and stir to cook through.

Turn off the heat and add the potato starch mixture to thicken the sauce. Return to low heat and pour in the beaten egg. Cook until the egg is semi-cooked, then turn off the heat.