Put the stock in a large saucepan and add the spring onion, ginger and garlic. Cover and bring to the boil, then simmer, covered for about 20 minutes. Scoop out the flavourings. Meanwhile, cook the eggs in a pan of simmering water for 7 minutes. Cool in cold water, then peel and halve. Cook the noodles in a saucepan of salted, boiling water for 4-5 minutes (or according to the packet instructions), separating them gently as they cook. Drain. Add the soy sauce to the broth and return to the boil. Add the mushrooms and cook for 1-2 minutes. Add the spinach and cook for 1 minute. Divide the noodles between four deep serving bowls and ladle over the broth. Top with the sliced char sui barbecue pork and half an egg each. Scatter over the beansprouts and sprinkle a little Shichimi togarashi over, if using, and serve. Alternatively serve the beansprouts and Shichimi togarashi on the side and let each person help themselves.
Tip: Buy char sui barbecue pork at Peking duck restaurants and shops. Ask for a piece rather than having it sliced, so you can slice it thinly yourself. Shichimi togarashi is a Japanese seven-flavoured spice mix available from Asian supermarkets.