In a medium saucepan, combine strawberries, sugar, and lemon juice. Bring to a boil over medium-high heat. Cook, stirring frequently, for 5 minutes. Reduce heat to medium-low, and cook, stirring frequently and mashing berries with a potato masher, until mixture thickens, 20 to 45 minutes. Remove from heat, and let cool for 30 minutes before transferring to a clean jar. Jam will keep refrigerated for up to 2 weeks.