On a lightly floured surface, roll out pastry into a rectangle about 16-18 inch long , and 12-14 inches wide. Place shrimp mixture in the centre of the pastry, spreading it out until it is the same width and length as the steelhead salmon fillet. Set salmon, flesh-side down, on top of the shrimp. Brush edges of pastry with egg wash, and then fold over fish and completely seal. Place seam side-down on a parchment paper-lined baking sheet. Brush top all over with egg wash. Make a few narrow slits on top to allow steam to escape. (Fish can be ready to this point several hours in advance. Wrap and store in the fridge until needed). Bake in a 375 degrees F oven 30-35 minutes. Let rest 10 minutes before slicing into portions with a serrated knife.
Note: Grainy mustard sauce is nice to serve with the fish. To make it, place 1/2 cup white wine in a pot, bring to boil, and then reduce by 2/3s. Add 2 cups whipping cream and reduce until sauce lightly thickens. Stir in 1/4 cup grainy Dijon mustard, season with salt and pepper, and serve.