Preheat the oven to 400F.
Rinse the bones and pat dry with a kitchen towel.
Lay the bones in a baking tray and roast in the oven until the edges are nicely browned.
Spread the spices in another baking sheet and roast in the oven until nicely charred.
Dump the bones and spices into a pot and pour in four liters of water.
Pour in a quarter cup of fish sauce.
Bring to the boil. Allow to boil, uncovered, for about ten minutes.
Lower the heat, cover the pan and simmer for about three hours or until the liquid is reduced by half.
Taste the broth occasionally and add more fish sauce, as needed.