Heat 2 tbsp of the oil in a large non-stick frying pan. Tip in the shallots and fry for 2-3 mins to soften, then add the mushrooms and garlic, and stir-fry over a high heat for another 3-4 mins, or until the mushrooms and shallots are golden and any liquid from the mushrooms has evaporated. Pour in the lemon juice – after a few seconds, that should have evaporated too. Remove from the heat, then stir in the watercress so it wilts in the warmth from the pan (see step 1). Stir in the dill and chives, and season with a little salt and pepper. Leave to cool.
Heat oven to 200C/180C fan/gas 6. Line a baking sheet with baking parchment. When the mushroom mix is cool, stir in the crème fraîche. Lay one of the filo sheets on the worktop with the short end facing you. Brush all over with a little of the remaining oil. Layer up 4 more of the filo sheets in the same way, brushing each with a little of the oil.
Lay one of the salmon fillets, skin-side up, across the width of the filo, positioning it about one-third of the way up. Season it with pepper. Spoon and spread the cooled mushroom mix over the top of the fillet (see step 2). Lay the other salmon fillet on top, skin-side down. Season again. Fold the short end of pastry nearest to you over the salmon, then bring the other end over to completely enclose the salmon (see step 3), lifting it so the join can tuck under it. Fold both pastry ends over as neatly as you can.
Brush the outside with a bit more of the remaining oil. Scrunch up the last sheet of filo, pressing it lightly on top in big folds (see step 4), then carefully brush with the last of the oil. Can be prepared 3-4 hrs ahead up to this point and chilled. Transfer the salmon parcel to the baking sheet. Bake for 25 mins until the pastry is crisp and golden. Check while it cooks 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 sit for 2-3 mins before slicing.