How to Make It
In a food processor, pulse the 1 3/4 cups of all-purpose flour with the kosher salt. Add the cubed butter and pulse until it is the size of small peas. Add the egg yolk and cold water and pulse until the pastry is moistened. Turn the pastry out onto a lightly floured work surface, gather any crumbs and pat into a disk. Wrap the disk in plastic and refrigerate until firm, about 45 minutes.
Preheat the oven to 375°. On a lightly floured surface, roll out the pastry to a 13-inch round. Ease the pastry into an 11-inch fluted tart pan with a removable bottom, pressing it into the corners and up the side. Trim the pastry 1/4 inch above the rim of the tart pan and use the excess to patch any holes or thin parts. Refrigerate the tart shell until chilled, about 15 minutes.
Line the tart shell with parchment paper and fill with pie weights or dried beans. Bake the shell for about 30 minutes, just until dry. Remove the parchment and weights and bake the crust for about 15 minutes longer, until golden. Let cool slightly, then transfer the tart pan to a large baking sheet.
Scatter the Époisses, apple and ham evenly in the tart shell. In a bowl, beat the eggs with the heavy cream, salt, white pepper and nutmeg. Pour the custard into the tart shell and bake for about 45 minutes, rotating the sheet halfway through baking, until the custard is just set and lightly browned on top. Transfer the tart to a rack and let cool for 30 minutes. Remove the ring, cut the tart into wedges and serve.