Sweetness and Light: Apple Vanilla Tart

My boy is suddenly into baking. It started with some muffins from the Hummingbird Bakery book (still the standard text for him), then he graduated to lemon-drizzle cake, then a simple plum cake, and so on (all enjoyed by his classmates at school, where he can get even more applause than at home). He found this recipe of mine, and made it on his own, amazing and delighting us. (Tip: The best apples  are Pippins, Gravensteins, Granny Smiths, or Winesaps, anything but Golden Delicious, which are too bland.) The