Rinse brisket and chicken bones in cold water. Put them together in a large pot and fill with 2 litres of cold water. Bring to the boil, skim the scum that rises to the surface and reduce heat to a steady simmer.

Put the oil and sliced onions together in a wok or frying pan and cook over a medium heat, stirring often, for 10 minutes or so until golden brown. This will give the broth a beautiful brown colour and rich flavour, but be careful not to overcook, which would make it bitter.

Drain the onions, discarding the oil, and spread them out on kitchen paper to absorb any excess oil.

Now add to the stock the onions, salt, the dark green ends of the spring onions (reserve the white parts for later), the garlic, ginger, cinnamon, star anise and coriander seeds. Simmer for about 2 hours or until the brisket is very tender. Remove the brisket to a bowl and allow to cool slightly. Strain the soup into a clean saucepan and discard the rest of the ingredients. Flake the brisket back into the pot of broth, discarding any fatty pieces. Now season the pho soup to your liking with the fish sauce and sugar.

Cook the rice noodles in a pot of boiling salted water until tender and drain.

To serve 

Divide the noodles into individual bowls (If using fresh noodles, heat briefly in the microwave or steamer.) Pour over the soup, slice the remaining white parts of the spring onions and sprinkle over the soup. Finish with a squeeze of lime juice.