I made 12 jars of this recently. I'll take one or two home to eat myself on toast with butter; the rest I'll use at work, smeared over tart shells, covered with almond frangipane and baked until golden.
Strawberries have a low pectin content, so it does not set as hard as other jams and is quite runny. If you prefer a firmer jam, add a third of a cup of redcurrant juice.
You will need sealable, sterilised jars to store the jam. Dishwashers sterilise everything, but if you don't have one, drop the jars into a pan of boiling water for 12 minutes then dry well before use.
1kg/2lb strawberries, hulled
1kg/2lb caster sugar
The zest and juice of 1 lemon
Put all the ingredients in a saucepan, place over a low heat and simmer gently until the sugar dissolves – always simmer very slowly to extract the natural pectin. Stir every now and then to prevent the fruit from catching and don't be afraid to top up with a little water if it becomes dry.
Once the sugar has dissolved, turn the heat to high and boil vigorously for 10 minutes. Once you think it might be ready, do the "wrinkle test". Place a spoonful of the jam on a saucer in the fridge for a few minutes to cool. Run a finger through the jam: if the surface wrinkles, it's ready. If not, return to the stove and boil swiftly. When done, rest the pan with the fruit in it for 10 minutes until the preserve is just setting around the edges. This helps to achieve even distribution of fruit and juice.
Spoon into jars, seal and allow to cool before storing in the fridge or cool place. It will keep well in the fridge for a few months.