Base: In a large bowl combine the flour and salt. Then add the butter and egg yolk. Knead well with your hands until smooth.
Sprinkle a little flour on a flat, floured surface. Transfer the dough and flatten it on the surface with a rolling pin (about 1 cm thick).
Transfer the dough to a round 25cm diameter springform cake tin. Having already lined the bottom and 2/3 of the side.
Bake for 20 minutes until the pastry is golden and crisp.
Custard: In a saucepan, add the condensed milk, 1 1/2 tins of milk and egg yolks. Mix well with the help of a whisk.
Cook over medium heat to thicken. When almost boiling add the cornflour mixed in the remaining milk, stirring continuously to avoid lumps until thickened. Remove from heat and set aside.
Apple Filling: In a heavy bottomed saucepan, add sugar and cook over low heat. When the sugar begins to melt and get the caramel colour, add the boiling water and stir well to dissolve all the sugar. Shortly after add the apple slices and stir carefully to caramelizes them. Cook for 2 or 3 minutes and remove from heat.
To Prepare: On top of the already baked tart base pour the custard and set well.
Then arrange the apple slices carefully on top of the custard and refrigerate for at least 4 hours before serving. Remove from the cake tin to present.