Make your pastry crust first by mixing flour, cold butter, sugar, and salt in a food processor until it looks like coarse sand. Add the egg and cold water one at a time, pulsing just until the dough comes together. Press it into a 9-inch tart pan and prick the bottom with a fork. Bake at 375°F for 12-15 minutes until it's lightly golden, then set aside to cool completely.

While the crust bakes, start your custard by whisking together egg yolks, sugar, and cornstarch in a medium saucepan. The mixture should be pale yellow and smooth—don't skip the whisking because lumps are basically impossible if you get this step right. Heat the milk and cream together in a separate pot until it's steaming but not boiling.

Slowly pour the hot milk mixture into your egg mixture while whisking constantly—this is the critical part that keeps eggs from scrambling. Keep whisking the entire time and don't pour too fast or you'll end up with chunks. Once everything's combined, pour it back into the saucepan.

Cook over medium heat, stirring constantly, until the custard thickens enough to coat the back of a spoon. You'll notice it'll look watery at first, then suddenly it'll thicken up fast around the 8-minute mark—don't panic if it looks thin initially. Cook for about 10-12 minutes total, and you'll know it's done when you run your finger across the spoon and it leaves a trail.

Remove from heat and stir in vanilla extract and salt, then press plastic wrap directly onto the custard surface to prevent a skin from forming. This is honestly the best trick I know for silky custard every single time. Let it cool for about 15 minutes, then pour it into your cooled crust.

Toss your sliced white peaches with lemon juice (this prevents browning and adds brightness), then arrange them on top of the custard in whatever pattern makes you happy. I like them in overlapping circles because it looks fancy, but honestly any arrangement works. This is where your tart gets its gorgeous face.

Chill the white peach custard tart for at least 2 hours before serving, or up to 8 hours if you're prepping ahead. You want the custard fully set so it holds its shape when you slice it. Careful not to let it sit in the fridge longer than overnight or the crust gets a little soft.