In a heatproof mixing bowl, whisk together the eggs, sugar, lemon zest, and freshly squeezed lemon juice until combined. Set over a saucepan filled with simmering water and cook while whisking continuously until thickened, about 10-20 minutes.

With the strained lemon curd still warm, add the small pieces of unsalted butter and whisk until fully melted and incorporated.

Pour the lemon filling into the fully baked tart crust, ensuring even distribution, and smooth the top with a spatula.

Cover the tart with plastic wrap and chill in the refrigerator for at least 4 hours, or ideally overnight, to allow filling to set.

Once set, remove from the fridge, garnish if desired, and slice carefully to serve.