Tom Yum Soup is probably one of the best-known Thai recipes. It's pretty easy to make and amazingly tasty. The soup buzzes with sweet, sour, hot and umami flavours.
Place the tomatoes in a large saucepan and mash them. Coarsely chop the garlic, ginger, lime leaves, lemon grass and onion and add them with the chicken stock to the tomatoes. Chop the coriander quite small and add the stems and roots (if you have them) and some of the leaves to the pan. Save some chopped leaves for later. Peel the langoustines: add the tails, shells and heads to the pan; put the flesh of the langoustines in the fridge until later. Squeeze lime juice into the pan, along with the tamarind liquid, sugar, fish sauce. Stir it all together and simmer for 20-30 minutes.
Give everything a good squish with a masher, and strain through a fine sieve. Return to the heat, add the peeled langoustines and the remaining coriander. Taste the soup - you're looking for a balance of sweet, sour and umami. If any of these is missing, add more sugar (sweet) or tamarind (sour) or fish sauce (umami). You may also need to add some salt.
Serve the soup in a bowl garnished with coriander, a twist of lime and a whole unpeeled langoustine.