Preheat oven to 200°C.
Prepare sachet of KNORR Classic White Sauce as per instructions then stir in the lemon juice and margarine to combine.
Take a sheet of pastry and place onto a board, brush with a little oil and cover with a second sheet.
Place a pile of rocket onto pastry edge nearest you and place the fish fillet on top.
Spoon over 2-3 tbsp of sauce.
Starting at the edge closest to you, roll fish in pastry, making sure the filling is enclosed completely.
Then fold in the pastry sides and continue to roll up like a sausage.
Place parcels on a baking tray and brush with a little olive oil, and sprinkle with sesame seeds.
Bake in a preheated 200°C oven for 15-20 minutes or until golden.
Serve with remaining sauce and salad.