Squeeze the fresh lemon juice and strain out any seeds. Add the lemon juice, sugar, and eggs to a heatproof bowl or the top of a double boiler. Whisk everything together until well combined and the sugar starts to dissolve. Place the bowl over gently simmering water and cook over medium heat, stirring continuously.
Keep stirring with a spatula until the lemon curd starts to thicken and coats the spatula. Add the butter and continue stirring until it has completely melted and the mixture is smooth. Remove from the heat and slowly strain the lemon curd through a fine-mesh strainer for an extra-smooth texture. Pour the warm lemon curd into a clean, sterilized jar and close with the lid.
Notes
Use fresh lemon juice. Freshly squeezed juice gives the curd a brighter, cleaner flavor than bottled lemon juice.
Cook it gently over a double boiler. Keep the water at a gentle simmer, and make sure the bottom of the bowl doesn’t touch the water. This helps the eggs cook slowly without scrambling.
Keep stirring while it cooks. Stir continuously, especially around the bottom and sides of the bowl, so the mixture thickens evenly.
Don’t wait until it looks very thick. The lemon curd should coat the back of a spatula when it is ready. It will continue to thicken as it cools.
Add the butter after the curd has thickened. Stir until the butter is completely melted and incorporated before removing the curd from the heat.
Strain the curd while it is still warm. This removes any tiny bits of cooked egg and gives you a smoother, silkier lemon curd.
Use a clean jar for storage. Pour the warm curd into a clean, sterilized jar, let it cool, then refrigerate it.