Make the dough: Put the flour and sugar in a bowl of an electric mixer fitted with the paddle, add cold butter cut into small pieces, egg and lemon zest . Process until large moist crumbs form(dough should hold together when squeezed).
Form a ball with the dough, wrap it in plastic and let it rest in the refrigerator for at least 30 minutes. Roll out the dough on a lightly floured surface. Cut out 6 circles of about 4-5 inches in diameter, using a cookie cutter. With each circle, coat 6 mini tart tins previously greased and floured. Prick their bottom with a fork , then put them in the freezer. Let mini tart tins rest there for at least 15 minutes.
For the filling
Prepare apples: Wash apples and start peeling them with a sharp knife or a peeler. Keep on doing that not only with apple skin, but also with the flash of the fruit. Put apple pieces (about 2 inches long) in a bowl and toss cinnamon powder, grated nutmeg, brown sugar and lemon juice. Stir gently and let them set asaid.
Prepare the cream: In an electric mixer whisk together egg yolk and sugar till they get light yellow and fluffy, then incorporate the flour. Stir again until mixture is smooth. Meanwhile heat up milk in a pan, then pour it into the mixture, stirring well to avoid lumps (mixer on low). Put everythingback into the saucepan, then cook the cream over medium heat for about 6-7 minutes, whisking constantly without stopping. The cream is ready when it begins to thicken, so you can remove it from the fire.
Set up and bake: Preheat oven to 350° F and take mini tart tins out from freezer. Put a good amount of cream in each tin (about 2 tbs) and level with a spoon, slightly wet if necessary. Take apple peels and place them on the pastry cream of each tart, starting to form concentric circles till reaching the center. Cook mini tarts for about 10 – 15 mins: they have to be slightly colored but not brown. Remove from oven and let them cool before serving