Preheat the oven to 200C/400F/gas mark 6.
Very lightly roast the langoustine shells in the oven for 5-10 minutes – you just want the shells to start to become fragrant and get a very light golden hue.
Meanwhile, put a large pot over a medium heat and add the butter. When warm, add the garlic, shallots, herbs and spices and sweat until the shallot is soft.
Add the lightly roasted shells to the pan and crush with the back of a wooden spoon to extract as much flavour as possible.
Add the vermouth and brandy to deglaze the pan. Turn up the heat and reduce by ½.
Stir in the tomato purée. Then add the stock and simmer for about an hour. (You want to be left with just under 1ltr of stock.)
When it’s done, strain the liquid – I use a mouli grater and crush the shells as I pass it, but you can also pass the mix through a colander, using a rolling pin to smash all the juice and other tasty morsels through the holes. After that, pass through a chinois or fine sieve.
Return the liquid to the heat in a small pan and add the cream. Bring up to a gentle simmer. Check the seasoning and add a squeeze of lemon, if you like.
Pour into bowls and serve immediately.