In a large saucepan, heat the oil and fry the onion for 1 minute. Add the haddock and half the sweetcorn, then fry for 5 minutes until the haddock is cooked through. Transfer a quarter to a bowl and set aside.
Meanwhile, in a food processor, whizz together the mashed potato, remaining sweetcorn, milk and stock. Add to the pan with the haddock, along with half the parsley. Simmer until heated through.
Divide between 4 bowls and top each with some of the reserved haddock, onion and sweetcorn. Top with the remaining parsley and season with freshly ground black pepper to serve.