Heat 2 Tbsp of the oil in a large non-stick skillet. Add shallots to skillet and saute for 2 to 3 minutes to soften. Then add the mushrooms and garlic, and stir-fry over high heat for another 3 to 4 minutes, or until mushrooms and shallots are golden and liquid from mushrooms has evaporated. Add the lemon juice and stir for a few seconds.
Remove skillet from heat, then stir in the watercress (or spinach) until it wilts from the warmth of the pan. Stir in the dill and chives, and set skillet aside to cool for several minutes.
Preheat oven to (200°C). Line a baking sheet with parchment paper; set aside.
When the mushroom mix has cooled, stir in the crème fraiche.
Lay one file sheet on a flat surface with the short end facing you. Gently brush the top of the filo sheet with a little of the remaining oil. Top with 4 more filo sheets in the same way, gently brushing each with a little of the oil.
Lay one of the salmon fillets, skin-side up, across the wide of the filo, positioning it about one-third of the way up. Season salmon with black pepper. Spoon and spread the cooled mushroom mix over the top of the fillet. Lay the other salmon fillet on top, skin side down. Season with black pepper. Fold the short end of the pastry nearest to you over the salmon, then bring the other end of the filo over, completely enclosing the salmon and carefully lifting the ends so you can tuck the long end under (if it's long enough). Fold both filo ends over as neatly as you can.
Gently brush the outside with a bit more of the remaining oil. Scrunch up the last sheet of file, pressing it lightly on top in big folds. Then carefully brush with the last bit of the oil.
It can be prepared to this point up to 3 to 4 hours in advance, and chilled until ready to bake.
Transfer the salmon parcel to the prepared baking sheet. Bake for 25 minutes until the outside of the pastry is crisp and golden. Check while it bakes, and if the top starts to brown too quickly, lay a sheet of foil very loosely over it.
Remove from the oven, and let the salmon parcel sit for 2 to 3 minutes before slicing into individual servings
FREEZING TIP: If the salmon wasn’t previously frozen, you can freeze it (unbaked) but might require additional baking time. Check after the initial 25 minutes of baking, and if needed, let it bake an additional 10 minutes.