Put the strawberries and lemon juice in a preserving pan or large, heavy-based stainless steel saucepan. Heat gently until the juices start to run, then use a potato masher to mash into the required texture, keeping some of the strawberries whole if you like. Continue cooking and stirring often until the mixture is reduced to a thick slush.
Add the sugar to the saucepan and stir continuously over a low heat until completely dissolved. Bring the mixture to the boil and boil steadily for 8–10 minutes, stirring frequently, until the setting point is reached (105 degrees C on a sugar thermometer).
Ladle the jam into warm, sterilised jars, then cover and seal. When cold, label, then store in a cool dark place. Use within 1 year of making. Once opened, store in the fridge and use within 1 month.