Sift the flour and dry mustard into a large bowl. Stir in the salt. Grate or cut the unsalted butter and shortening into the flour. Mix together the cold water and white vinegar. add the liquid 2 tbsp at a time, to the flour and butter, pulling the dough together. Mix briefly to form a ball. Divide the dough into four equal parts . Wrap tightly in plastic wrap. Refrigerate or freeze until needed Buy the puff pastry.
Preheat oven to 450F. Roll out savoury short crust and puff pastry into two 12" rounds, ¼ " thick. Refrigerate puff pastry on a baking sheet lined with parchment paper. Line a 10" pie plate, at least 2 " deep, with the savoury short pastry crust and refrigerate for at least 1 hour. Heat olive oil in a large skillet over medium high heat. Add the garlic and saute for 3 mins. Add vegetables Cook for 12 minutes until the mixture is lightly browned and potatoes are soft. In a small mixing bowl, sprinkle the flour on the cream and whisk to combine. Deglaze the skillet with the dry Riesling. Add the cream and the flour mixture. Heat to just under the boil, stirring constantly. Cook until very thick, about 3 minutes. Add thyme, Italian parsley and tarragon, stirring to combine. Fold in the raw seafood. Season with salt and black pepper. Pour the filling into the pastry shell. Top with the puff pastry and crimp the edges to seal. Cut 6 small cross-shaped slits in the pastry to allow steam to escape. Whisk together the egg and the grainy mustard. Brush surface of the pie with the egg wash. Bake for 15 minutes at 450F. Reduce heat to 350F and bake 45 minutes more until golden brown and puffed.