Thai oxtail soup

Put the oxtail in a pot with plain water.

Smash the top half of the lemongrass and add it with lime leaves to the pot; save the bottom half for later.

Season with fish sauce and simmer gently for two and a half hours, topping up with water and skimming the scum and fat as it goes.

Fish out the spent herbs. Optionally chill overnight after this stage and scrape the set fat off the top.

Add the onion and chopped tomatoes and cook another 30 minutes until the onion is soft and the tomatoes disintegrate.

Add the reserved lemongrass bottoms, ginger slices, torn and bruised lime leaves and halved Thai chilies; simmer 15 to 20 minutes.

Turn off the heat, taste and adjust with more fish sauce or salt.

Add extra tomato chunks and simmer one minute to soften, or let residual heat soften them if serving later.

Finish with cilantro and serve with lime on the side to squeeze in to taste.