Cut into, but not right through, salmon and then add a knob of butter. Squeeze lemon juice over each salmon fillet. Place salmon on pastry so that the pastry can be folded over. Seal with water round edges to make sure each parcel is sealed.
Grease a baking tray or line with parchment. Place salmon parcels on tray, then brush the tops with beaten egg.
Place in a preheated 180 C / 160 fan / Gas 4 oven and bake for 20 minutes, or until pastry is golden.