For the beef, cut the beef slices into 3 cm (1¼ in) strips. Place the beef, soy sauce, sugar and mirin in a saucepan. Sauté, stirring regularly, over a low to medium heat. Add the dashi and let it simmer on a low heat for 5 minutes.
To make the soup, put all the ingredients in a saucepan and bring to the boil.
Cook the noodles according to the instructions on the packet. Drain the noodles and divide between four bowls. Pour the soup over the noodles. Top with the beef, garnish with the spring onion and serve immediately.
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