Begin by rolling out the thawed puff pastry on a floured surface until it's about ⅛ inch thick. Cut out circles that fit into your muffin tin.

Press the pastry circles into the greased muffin tin, shaping them into shallow tart shells. Chill in the refrigerator while preparing custard.

In a saucepan over medium heat, combine heavy cream and whole milk until it simmers. Whisk egg yolks, sugar, and flour until smooth in a separate bowl.

Slowly pour the hot milk mixture into the egg mixture, whisking to prevent curdling. Return to heat until thickened, about 5-7 minutes.

Remove from heat, stir in vanilla extract and lemon zest, and let cool slightly before filling the pastry shells.

Preheat oven to 400°F (200°C) and carefully pour custard into the chilled pastry shells, filling just below the top.

Bake for 15-20 minutes until edges are crispy and custard is slightly trembling in the center.

Let cool in the tin for about 10 minutes, then transfer to a wire rack to cool completely.