Place the mashed strawberries and sugar in a large, heavy-bottomed saucepan and stir to combine. Heat the mixture over medium-high heat, stirring frequently, until the sugar has dissolved. Add the lemon juice and stir to combine. Bring the mixture to a boil, then reduce the heat to medium-low and simmer, stirring occasionally, for about 30-45 minutes or until the jam has thickened and reached the desired consistency.

To test if the jam has set, place a small amount on a chilled plate and let it cool for a few minutes. Run your finger through the jam. If it leaves a clear path and doesn't fill in with liquid, the jam is ready. Remove the jam from the heat and let it cool slightly. Skim off any foam that has formed on top, if desired.