1 - step 1 Pour 1.

2 - 3 litres water into a large saucepan over a high heat.

3 - Add the onion, tomato, chilli, galangal, lemongrass, prawn heads and chicken stock cube.

4 - Stir and bring to a boil, then reduce the heat to medium and simmer for 20 mins until the liquid has reduced.

5 - step 2 Carefully strain the hot broth into a large heatproof bowl or jug, then discard the prawn heads.