Hull each strawberry by cutting a cone shape into the top of the berries with a small knife and removing the stalks. Cut them into halves or quarters, depending on the size. Tip into a large pan with the sugar and toss well. Leave to macerate for 2 hours. The sugar will draw moisture from the strawberries, making sure they don’t disintegrate too much and keep their vibrant colour. Put two saucers in the freezer – this will help you tell if the jam is set later.
Warm the mixture over a low heat, stirring gently for 5-10 minutes or until the sugar has dissolved. Turn up the heat and boil for another 5-10 minutes, stirring regularly to avoid it catching on the bottom, until the jam has noticeably thickened.
Remove one of the saucers from the freezer and spoon a little of the jam onto it. Leave to set for 30 seconds-1 minute. Once cooled, run your finger gently through the jam. The surface should wrinkle and your finger should leave a clear, prominent trail that doesn’t fill back in. Alternatively, test to see if the jam has reached 105C on a sugar or digital thermometer.
Skim off any froth or scum that has risen to the top with a spoon. Ladle into clean, sterilised jars and top with a circle of wax or baking paper. Screw on the lids and label each jar clearly. This jam will keep for 1 year in a cool, dark place. Chill once opened.