Peel, core and roughly cut the apples into about 8 pieces. Put in a saucepan and immediately cover with ½” of cold water.
Add about a teaspoon of sugar. Bring to the boil and simmer until the apples collapse – about 10-15 minutes.
Keep a watch and be ready to top up with boiling water – you don’t want the apples to boil completely dry and catch but you don’t want them swimming in water either – if there is more than a tablespoon left, strain it off and discard.
Purée the apples in a liquidiser and add the lemon juice and, importantly, zest.
Add the butter while the mixture is still hot and whizz again. Season with salt and pepper and taste for sweetness – I like it quite sharp but how much sugar it needs depends so much on the apples.