Make sure all the ingredients are prepared and ready before cooking the udon noodles.
Cook the udon noodles according to the packet instructions, then drain.
Quickly divide the hot noodles among four bowls. Top each with an egg yolk and a quarter of the spring onion and katsuobushi, and drizzle with the Mentsuyu or soy sauce. Serve immediately. Stir well with chopsticks as you e