In a bowl, mix the flour, sugar, and salt. Add grated cold butter and gently work it into the flour with your fingertips until it resembles coarse crumbs. Stir in the crushed biscuits. In a small bowl, whisk the egg yolk with cold water. Add to the flour mixture and combine just until a dough forms. Wrap the dough and chill for at least 1 hour.
Roll the chilled dough thinly. Cut into rounds and press gently into tartlet molds or a muffin tin. Chill the lined molds for 10–15 minutes. Blind bake at 180°C for 12–14 minutes, then remove weights and bake for another 4–5 minutes until golden. Cool completely.
Lemon Curd method
In a small saucepan, combine the lemon zest, lemon juice, and sugar. Add the egg yolks and whole egg. Whisk everything together until smooth and fully combined.
Place the saucepan over low to medium heat. Cook gently, stirring constantly with a silicone spatula or wooden spoon, making sure to scrape the bottom and sides. Do not let it boil — the curd should thicken slowly. This may take 6–8 minutes. Once the mixture coats the back of a spoon (nappe consistency), remove from heat.
Let it cool slightly (30 seconds to 1 minute), then add the cold butter cubes, stirring until fully melted and incorporated. Strain through a fine sieve into a clean bowl to remove any zest or bits of cooked egg.
Let cool completely, then fill your tart shell or tartlets. Chill for at least 2 hours until set.