Layer the strawberries and sugar in a large bowl, finishing with a layer of sugar. Cover the bowl and leave for 24 hrs. Meanwhile, put a couple of saucers in the freezer to chill.
Next day, the juices will have been drawn out from the fruit, leaving you with a bowl of berries in a sugary pink syrup. If this hasn’t happened, give the berries a quick stir, then leave for a few more hrs.
Tip the mixture into a preserving pan (or a large pan with wide, sloping sides) and warm to dissolve the remaining sugar. Once dissolved, bring to the boil, stirring frequently until bubbling.
Pour in the lemon juice, return to the boil, then boil hard for 10 mins, stirring occasionally. If the surface of the jam starts
to look a little scummy, add the butter. Take from the heat, then drop a little jam onto a chilled saucer. Let it cool a little, then push your finger through the jam. If the jam parts, leaving wrinkled ripples, it is ready. (Turn to p23 for more detail on testing when jam is ready.) Allow the jam to cool for 30 mins, then pot into sterilised jars and, when the jam is cool, label the jars. Keeps for 1 year – store in the fridge once open.