Place the flour, salt, and fresh dill in a large food processor and blitz on low until combined.
With the food processor running, feed the butter into the machine, one cube at a time. Add the mustard and pulse until the mixture resembles a coarse meal. Set the food processor to low and stream in the cold water. Blizt until a dough forms.
Transfer the dough to a floured surface and form into a ball. Press the ball into a disc and wrap tightly in plastic wrap. Chill for at least 30 minutes or overnight.
Place the chilled dough on a piece of parchment paper dusted with flour. Leave the dough to thaw for 15 minutes. Roll the dough out and place it on top of the cooled filling. You can do a lattice pattern or simply place the pastry on top, just make sure to cut a vent in the center.
Transfer the finished pie to the fridge and let chill for 30 minutes. While the pie is in the fridge, preheat the oven to 425°F.
Once the oven is up, whisk an egg and 1 tbsp of water together in a small bowl. Using a pastry brush, brush the surface of the pie with the egg mixture and place it in the oven. Bake the pie for 35-40 minutes or until bubbling and golden brown. Let stand for 10 minutes before serving.