Season salmon fillets on both sides with salt, pepper and dill. Sprinkle flesh side with lemon juice.
Heat a med skillet over med high heat. Add oil and butter. When hot, add salmon, skin-side down. Brown well, about 3 minutes. Turn and sear top side, about 2 minutes. (Note: Salmon should be rare, because it will cook fully in the pastry.).
Remove from pan and refrigerate untl completely cooled, about 1 hour. Save pan and drippings to make dill sauce.
For the Filling:
Heat a new med skillet over med high heat. Add butter and oil. Saute onions until tender, about 5 minutes.
Add mushrooms, spinach, salt and pepper. Saute until mushrooms are cooked through, about 5 minutes. (Note: There should be no moisture but, if there is, make sure it evaporates.).
Place in refrigerator to cool.
Thaw pastry for about 20 minutes.
Preheat oven to 400°F.
On a lightly floured surface, roll pastry out into a 14-inch square, about 1/8 inch thick. Cut into 4 equal squares. Place about 1/4 cup of spinach mixture in the middle of each square,then top with a piece of salmon, skin side up. Brush the edges of the pastry with some egg wash . Bring up opposite corners of pastry around the top of the salmon. Pinch the edges together to seal.
Place the pastries on an ungreased baking sheet, seam-side down. Brush the tops with the remaining egg wash.
Bake until golden brown, about 25 minutes. (Note: This is a good time to make the dill sauce in the salmon drippings.).
Remove pastries from oven and let stand 5 minutes.
For Creamy Dill Sauce.
Heat med saucepan over med high heat - or use the skillet in which the fish was cooked.
Add wine and lemon juice; bring to a boil.
Add cream. Simmer until well-combined and cream has reduced slightly.
Stir in dill and parsley. Taste for seasoning.
Spoon some of the sauce on each plate, then top with salmon pasty. Any remaining spinach-mushroom mixture can be heated and served on the side.