Wash and slice the strawberries, removing the hulls and green tops.
Add them to a saucepan, and mix the sugar in well.
Stir continually over medium heat, and bring the strawberries to boil.
Once boiling, add the lemon juice and zest (if using).
Boil for about 15 minutes, or until the jam reaches 220F.
Stir often, to make sure the jam doesn’t burn.
Pour into a jar and allow to cool.
Cover and refrigerate.
Use within about 2 weeks (if it lasts that long).