In a medium saucepan, combine the 3 large eggs, 1 cup of granulated sugar, and a pinch of salt. Whisk together until fully combined and slightly frothy.
Stir in the 1/2 cup of fresh lemon juice and the zest of 1 large lemon. Make sure everything is well incorporated.
Place the saucepan over low heat. Continuously whisk the mixture, ensuring it doesn’t stick to the bottom of the pan. Cook until the mixture thickens enough to coat the back of a spoon, about 10-15 minutes.
Once thickened, remove the saucepan from heat. Pour the lemon curd through a fine-mesh strainer into a clean bowl to remove any cooked egg bits for a smooth texture.
While the curd is still warm, add the 4 tablespoons of diced unsalted butter. Stir until the butter is completely melted and incorporated into the curd.
Let the lemon curd cool to room temperature, then transfer it to a heatproof jar or container. Store in the refrigerator for up to two weeks.
