This is a very sharp, concentrated preserve which goes wonderfully with fish, especially Rösti Crab Cakes. It is also good with Baked Thai Red Curry Chicken – but because it's so strong very little is needed. To sterilise the jar, wash it thoroughly in warm soapy water, rinse and put it in a cool oven to dry and get warm.
You need to begin this recipe the night before, as the limes have to be salted to extract their bitterness.
To do this, slice off and discard the ends of six of the limes, then cut into slices (skin and all) about 1/8 inch (3 mm) thick. Now spread a layer of kitchen paper all over the non-metallic tray and lay the slices of lime on it in a single layer with no overlapping. Sprinkle half the salt over the limes, leave them like that for a few hours, then turn them over and sprinkle the rest of the salt over the other side. Cover loosely with another layer of kitchen paper and leave in the fridge overnight.
Next day rinse the lime slices in a colander, sluicing and turning them under a cold running tap until all traces of salt have been washed away.
Now transfer them to a saucepan with just enough water to cover them, then simmer very gently for about 30-45 minutes or until the slices are very tender (watch this carefully after 30 minutes are up, otherwise it's possible that they can overcook and turn to mush: the slices need to be tender but still intact).
Now squeeze the juice from the other limes – you need 7 fl oz (200 ml) in all and sometimes this can mean six limes, sometimes seven or eight, depending on their size and juiciness.
Place the juice in a saucepan along with 4 fl oz (110 ml) water, the sugar and spices, stir over a gentle heat until the sugar has dissolved completely, then simmer very gently for 25 minutes without a lid. The liquid will reduce slightly, but do keep an eye on it as it mustn't boil rapidly or it will tend to caramelise.
Then remove the syrup from the heat and take out the spices using a draining spoon. Now add the drained lime slices to the syrup and pour the whole lot into the warmed sterilised jar.
Seal it down, label when cold and keep for a month before eating.