Wash the jam jars and rinse; place them top down in the oven (gas 1, 140°C, fan 120°C); warming the sugar at the same time in the oven, while you start making the jam.
Wash and hull the strawberries and slice in two, checking for any damages, which must be discarded.
Place the strawberries in a large heavy pan with the lemon juice. Cook over a low heat for 20-30 minutes until soft. Stir occasionally to prevent sticking.
Remove the pan from the heat, add the warm sugar and stir until fully dissolved. Return to a high heat and bring to a rolling boil, stirring frequently to prevent burning.
Test to see if the jam is at setting point after 15 minutes. Either use a sugar thermometer and allow the jam to reach 105°C; or chill a saucer in the freezer, pour on a drop of jam, allow to cool. If you push the jam with your finger, it should wrinkle. If it is still liquid it is not yet ready to set; boil for another five minutes.
When the jam has reached setting point, take the pan off the heat, and remove any white residue from the top with a slotted spoon.
Carefully pour into sterilised pots and cover with waxed discs and lids. Cool and keep in the fridge.
You can sterilise jam jars in the dishwasher.
See more breakfast recipes