Prepare the Crust. In a food processor, pulse the flour, sugar, and butter until the mixture resembles coarse crumbs. Add the egg yolk and 1 tablespoon of cold water, pulsing until the dough comes together. Add an additional tablespoon of water if needed. Press the dough into mini tart pans and refrigerate for 30 minutes.
Bake the Crust. Preheat the oven to 375°F. Prick the bottom of each crust with a fork. Bake for 12-15 minutes or until lightly golden. Let cool.
Make the Lemon Filling. Whisk together eggs, egg yolks, sugar, lemon juice, and lemon zest in a saucepan over medium heat. Stir in the melted butter. Cook, stirring constantly, until the mixture thickens, about 5-7 minutes. Strain through a fine sieve.
Fill and Chill. Spoon the lemon filling into the cooled tart shells. Refrigerate until set, about 2 hours.
Serve. Garnish with whipped cream, lemon zest, and fresh berries if desired before serving.