Cut open the mentaiko and remove the contents. Trim the mitsuba roots and cut into 2cm pieces.

Beat the egg in a bowl until smooth. Place the udon on a microwave-safe plate, cover with plastic wrap, and heat according to package instructions.

In a pot, combine the seasoning ingredients and heat over medium heat. Once boiling, add the potato starch mixture and stir until thickened.

Add half of the mentaiko to the sauce and mix. Gradually pour in the beaten egg, stirring gently until semi-cooked, then turn off the heat.