Preheat the grill to the medium-high setting.
Place a frying pan over a high heat. When hot, add the star anise, cloves and coriander seeds and toast until aromatic. Transfer to a plate and set aside.
Put the onion and ginger on a grill pan and place under the grill, turning from time to time, until soft and slightly caramelised, but lightly charred on all sides. Leave to cool a little, then peel the onion and ginger and discard the skins.
Put the onion, ginger and toasted spices into a large, lidded pot. Add the stock and bone marrow and bring to a steady simmer. Cover, lower the heat a little and simmer for 30 minutes to allow the flavours to develop and infuse the broth. Add the fish sauce for the last 10 minutes of the cooking time.
Strain the broth through a sieve into a clean saucepan and return to the boil.
Meanwhile, cook the noodles according to the packet instructions, drain and refresh under cold running water to stop the cooking process.
Put the noodles in deep bowls and top with the sirloin. Ladle the hot broth over the top. Add the beansprouts, spring onions, chilli, herbs and a wedge of lime at the table.