Heat the oven to 200C/fan 180C/gas 6. Whizz the watercress with enough of the stock to make a paste. Lay out both sheets of filo and put a piece of salmon at the end of each sheet in the centre.
Spread the watercress paste over the salmon, then fold and roll up the parcels and put them on a baking tray. Bake for 20 minutes, or until the pastry is browned and crisp. Serve with a green salad and lemon wedges.