Pre-heat the oven to 230C/450F/Gas MK 8.
Put the potatoes into boiling, salted water and bring back to the boil for 2 minutes.
Carefully add the eggs to the pan and cook for a further 8 minutes until the eggs are hard boiled and the potatoes cooked.
Drain the potatoes. Remove the eggs and cool under cold water, then shell and quarter them.
In a separate pan, slowly fry the onion in a little olive oil for about 5 minutes, then add the cream and bring just to the boil. Remove from the heat and add the cheese, lemon juice, mustard and parsley.
Put the fish, prawns, sweetcorn and eggs into an ovenproof dish. Pour over the sauce and gently mix together.
Mash the potatoes with a little olive oil, seasoning and nutmeg. Spread over the top of the fish mixture and bake in the oven for 25-30 minutes until the potatoes are golden.