Preheat the oven to 190ºC / 375ºF / Gas Mark 5.
Cut the haddock into bite-sized pieces and toss in cornflour to coat evenly.
Place the fish (and prawns, if used), sweetcorn and peas in an ovenproof dish. Beat together the milk, fromage frais and seasonings, then pour into the dish.
Mix together the breadcrumbs and grated cheese, then spoon evenly over the top.
Bake for 25-30 minutes, or until golden brown.
Serve hot with fresh vegetables.