Preheat oven to 180°C.
Lightly grease a 1.5 Litre ovenproof dish.
Put potatoes and a pinch of salt in a saucepan, cover with boiling water and cook for 15-20 minutes, or until tender.
Place Stork margarine and coriander into a food processor and process until fine.
In a medium pot, mix together 300 ml milk and sachet of KNORR Classic White Sauce.
Add the bay leaf, onion and parsley sprigs and, whisk until the sauce thickens.
Remove from heat and discard the bay leaf, onion and parsley.
Stir in the fish, prawns, chopped parsley, chives, lemon juice and seasonings, spoon into the ovenproof dish.
When the potatoes are tender, drain well and mash with the coriander-margarine mixture and low-fat milk until smooth.
Season with salt and pepper.
Spoon the mashed potato evenly over the creamy fish mixture.
Bake for 25 minutes or until browned.