Put strawberries and lemon juice in a large saucepan. Cook, stirring occasionally, over low heat until juices are released, about 40 minutes. Stir in sugar.
Bring to a boil over medium heat. Cook, stirring occasionally, until mixture registers 210 degrees.on a candy thermometer, about 15 minutes. Let cool completely; skim foam from surface with a spoon. Preserves can be refrigerated in an airtight container, up to 2 months.