Tart Pastry: Work lightly together the pastry flour, butter, egg yolk, a pinch of salt and cold water. Chill the dough for 1 hour, or until needed.
Roll out half the dough to line the pie pan. Prick randomly with the point of knife and crimp the edge with the tines of a fork. Save remaining pastry for another pie.
Filling: Scatter diced ham on the crust.
Sauté sliced onions in butter until they are soft, but not brown; spread over ham.
In a saucepan, beat four eggs with a good pinch of salt, cayenne and nutmeg. Gradually add hot milk, beating continually with a wire whisk. Continue to beat the mixture over a low fire until the custard begins to thicken. Pour it into the tart shell and bake at 375° F for 30 minutes, or until custard is set and the top is golden. Serve hot.Makes 6