Slow-frying leeks in butter or oil brings out their silky sweetness. They’re good alone, or as an unctuous base to other recipes. The key is patience and a low heat, to avoid ending up with a nasty mixture of burnt and raw leeks.
Sweat sliced leeks on a medium–low heat, stirring frequently, until they are completely tender but not coloured – they need a good 10–15 minutes. To be sure they’re done, taste a piece – it should be sweet and soft. Season with a little salt and pepper and a tiny dash of vinegar if you need to balance the sweetness.