In a medium saucepan, combine sugar, lemon juice, and lemon zest.
Whisk in the eggs until fully incorporated.
Place the saucepan over medium heat.
Stir constantly until the mixture thickens (about 10-15 minutes).
Remove from heat and strain through a fine-mesh sieve into a bowl.
Add the cubed butter to the warm mixture.
Stir until the butter melts completely.
Pour the lemon curd into sterilized jars.
Allow to cool at room temperature.
Refrigerate for up to two weeks.