1. Process strawberries in a blender until smooth; press through a wire-mesh strainer into a 3-qt. saucepan, using back of a spoon to squeeze out juice; discard pulp. Stir in sugar.
2. Whisk together lemon juice and cornstarch; gradually whisk into strawberry mixture. Bring mixture to a boil over medium heat, and cook, whisking constantly, 1 minute. Remove from heat. Place plastic wrap directly on warm jam; chill 2 hours or until cold. Refrigerate in an airtight container up to 1 week.