Sift the flour and sugar together into a large mixing bowl. Coat the butter with flour from the bowl to make it easier to handle, and grate it directly onto the flour, using the coarse blade of a cheese grater. As you grate the butter, mix the flakes together with the flour occasionally, using your fingertips, before grating more.
Add the sugar and lemon rind. Using two round-bladed knives, cut the butter into the flour until the mixture resembles cornmeal.
Mix the egg lightly with a fork and pour over the flour mixture. Using the fork, mix the egg into the flour mixture until well distributed. Then, using your hand, pull the dough gently together into a ball. Knead lightly and briefly on a floured board until the dough forms a cohesive mass. Pat into a flat round and wrap tightly in Saran wrap / plastic wrap / clingflim. Refrigerate for at least an hour before using.
Roll the dough out just thin enough to be about an inch and a half wider than the 10-12" tart pan, and either fold it in two for the move into the pan, or drape it over the rolling pin and move it that way. Carefully tuck the dough down so that it is flat both against the pan's bottom and the fluted sides. Press it gently against the fluting: then trim it off flat by rolling the rolling pin over the top edge. Prick the bottom of the crust-to-be with a fork.
Preheat the oven to 375°F.
Sprinkle the bottom of the pastry-lined pan/tin with the ground almonds. Combine the cinnamon, sugar and lemon zest in a small bowl.
Peel, core and quarter the apples. Slice them very thin (you may want to use a mandoline-type vegetable slicer for this), toss them together with the cinnamon-sugar-zest mixture, and then arrange them in concentric circles on top of the ground almonds. Flake the butter on top.
Mix the cornstarch well with about three tablespoonsful of the cream: then add this to the rest of the custard ingredients and mix well.
Put the pastry in the oven and bake for fifteen minutes.
When this time has elapsed, stir up the custard ingredients well one more time, then pour enough custard into the pastry to fill it to the top.
Close the oven and bake for another 30 minutes. (If the pastry was showing too much browning at the 15-minute stage, you might want to decrease the heat just a little.)
At the end of the 30 minutes, wiggle the pan a little to make sure that the custard's set. If it is, remove and allow to cool on a rack. If it isn't, give it another five or ten minutes, but no more. The custard usually puffs up while baking: when it collapses after you remove it from the oven, don't worry -- it's supposed to do that.
The apfelwähe can be served hot, warm, or cold. Add whatever sides you prefer with an apple dessert -- pouring cream, more custard, a sharp cheese.