Peel and grate the carrot and yam.

Cut open the mentaiko and extract the roe.

In a pot, combine grated carrot, yam, white dashi, and water. Heat until boiling, then turn off the heat and mix in soy milk and two-thirds of the mentaiko. Heat gently over medium heat until just before boiling.

Place frozen udon in a microwave-safe bowl, cover loosely with plastic wrap, and microwave according to package instructions. Stir to loosen.