Defrost the udon noodles by immersing them in a bowl or pan of just-boiled water. Stir gently until disentangled and softened. Drain well and set aside.
Combine the soy sauce, mirin and grated ginger in a small bowl and set aside.
Heat the vegetable oil in a large frying pan (about 30cm in diameter is good) over a high heat. Add the pork belly slices and fry, stirring, until nicely browned. Add the prawns and squid rings after about 2 minutes and halfway through cooking the pork.
Add the onion, carrot and cabbage to the pan and stir-fry for 3 minutes more.
Add the cooked udon noodles and beansprouts, then stir-fry with the vegetables for 1 minute.
Add the dashi powder and the soy sauce, mirin and ginger mixture and stir-fry just until all the ingredients are coated in the seasonings.
Finally, stir in the ground white pepper.
Divide the udon noodles onto plates, and serve topped with spring onions/ scallions, katsuobushi and aonori.