By hand or in a food processor, mix flour, butter, and salt until crumbly.
After that transfer flour mixture to a medium bowl. Add the ice water and mix with a fork until stiff dough forms.
Wrap in plastic wrap and chill in the refrigerator for 30 minutes.
Preheat oven to 375°F. Line a baking tray with a piece of foil, and brush over a little oil.
Mix the softened butter with chopped fresh herbs and the lemon zest.
Cut the salmon fillet into 2 equal-sized pieces and season with salt and pepper.
Spread the butter mixture over one side of the fillet (on the boned side) and the mustard on the other fillet.
Sandwich the two pieces together (skin side out).
Take the pie crust dough out of the fridge. On a lightly floured work surface, roll out the pastry and enclose the salmon, carefully sealing the edges.
Glaze it with the beaten egg yolk and score a criss-cross pattern on top of the pie, using the back of a knife.
Bake in the oven for about 20 minutes, or until the crust is golden brown. Then remove and allow to cool slightly before serving. Enjoy!