Cook the pasta in boiling water with half the oil until just soft - about 9 minutes. Drain and refresh under fresh cold water, drain again and sprinkle with a little olive oil to stop it sticking together, set to one side.
Poach or steam the fish until cooked and set aside to cool.
For the sauce, melt the margarine, add the flour and cook until it changes to a light sandy colour.
Add the ilk gradually, stirring continuously until smooth then cook until it starts to thicken.
Add the onion and continue to cook for approximately one hour. Add the parsley and adjust the consistency to be able to coat the back of a spoon.
Flake the fish, checking and removing any small bones. Pour the pasta into a large ovenproof bowl followed by the sauce, fish and peas and stir thoroughly. Adjust the seasoning.
Divide into greased containers, sprinkle with grated cheese and breadcrumbs and bake in the preheated oven until golden and bubbling - about 20 minutes.