Tip the beef stock along with 500ml of water into a large saucepan. Sit the onion and ginger in a frying pan over a high heat and char on all sides, around 3-5 mins (you can also do this under your grill). Once charred, add to the beef stock. In the same pan, toast the spices for 2-3 mins and once they begin to smell fragrant, add them to the beef stock as well. Bring the stock to the boil, then turn to a simmer and cook for 30mins before straining.