In a heatproof bowl, combine two large eggs with two egg yolks, ¾ cup of granulated sugar, one tablespoon of lemon zest, and ½ cup of freshly squeezed lemon juice. Whisk until well blended.

Place the bowl over a saucepan filled with simmering water, creating a bain-marie. Keep whisking constantly for about 10-20 minutes until the mixture thickens.

Remove the bowl from heat and strain the lemon curd through a fine mesh sieve into a clean bowl.

While the curd is still warm, add the unsalted butter and whisk until fully melted and incorporated.

Pour the cooled lemon curd into your fully baked tart crust and smooth the top.

Refrigerate the filled tart for several hours or overnight to firm up before slicing.