Mix the strawberries and jam sugar (and vanilla seeds, if desired) in a saucepan over low heat. Cook for about 2 hours, stirring often. Add the lemon juice. Briefly puree with a hand blender.
Stir for about 5 minutes and bring to a boil, skimming off the foam.
Pour the jam into hot rinsed jars.
Flip the jars upside down and let cool for about 15 minutes. Flip right side up, cool completely and serve.