In a large bowl, combine the flour and salt. Cut in the cold, cubed butter until the mixture resembles coarse crumbs.
Gradually add the cold water, mixing until the dough comes together. Form the dough into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.
Heat the vegetable oil in a large skillet over medium heat. Add the chopped onion and garlic, and cook until softened and translucent.
Add the ground pork and veal to the skillet. Cook until browned and cooked through, breaking up the meat with a spoon.
Stir in the Riesling wine and cook for a few minutes until the liquid has reduced slightly.
Stir in the heavy cream, parsley, and ground nutmeg. Season with salt and pepper to taste. Remove from heat and let the filling cool slightly.
Preheat your oven to 375°F (190°C).
Roll out the chilled dough on a lightly floured surface to fit a 9-inch (23 cm) pie dish or tart pan. Transfer the dough to the pan, pressing it into the edges and trimming any excess.
Pour the filling into the prepared pastry shell.
Roll out the remaining dough and use it to cover the filling. Seal the edges and cut a few slits in the top crust to allow steam to escape.
Brush the top of the pastry with the beaten egg.
Bake in the preheated oven for 35-40 minutes, or until the crust is golden brown and the filling is hot and bubbly.
Allow the Rieslingspaschtéit to cool slightly before slicing. Serve warm or at room temperature.
Enjoy your meal!