Put the onion in a large bowl with half the chilli, coriander, ginger, mince and some seasoning. Use your hands to bring all the ingredients together until well combined. Divide the mixture roughly into four portions. Cut each quarter into four or five smaller portions and then, using your hands, shape the mixture into meatballs. Heat the sunflower oil in a large frying pan and fry the meatballs, do this in batches if needed, for about 5-8 minutes until golden all over. Meanwhile, pour the beef stock into a large saucepan and bring to a gentle simmer. Add the remaining chilli, the noodles, carrots, broccoli and meatballs to the broth. Bring the broth to the boil and simmer for 4 minutes until the noodles and vegetables have softened and the meatballs are cooked through. Divide the noodles, vegetables and meatballs between four bowls and ladle over the broth. Scatter over the spring onions and a little coriander to garnish. Serve with sweet chilli sauce on the side, to drizzle over, if you like.