Before you start sterilise 2 jam jars by washing them thoroughly in hot soapy water and rinsing in warm water.
Preheat the oven to 140°C (fan 120°C/Gas 1), and stand the jars upside-down on a rack in the oven to dry for about 30 minutes. Alternatively, after rinsing, dry the jars in the microwave for 1 minute – they must be wet, but not filled with water or they will explode.
Combine the strawberries and sugar in a medium-sized saucepan and bring them to a simmer over a medium-high heat, stirring frequently. Reduce the heat to medium and continue to simmer for 1 hour or until thick, stirring occasionally. Remove from the heat and stir in the lemon juice.
Pour into the warm jars, seal and allow to cool. Store in the refrigerator. The jam will keep up to 6 months.