Put the swede and potatoes in a saucepan and bring to the boil. Cook for 15–20 minutes until tender, drain and mash. Season and set aside.
Meanwhile, put the cheese and stock in a small saucepan, whisk until fairly smooth and heat gently until melted and combined. Add the cornflour mixture and cook until thickened. Remove from the heat and season.
Stir the fish and peas into the cheese mixture and place in a medium ovenproof pie dish.
Spoon the mash over the fish mixture and bake for 25–30 minutes, or until lightly browned. Garnish with chopped herbs, if desired, and serve immediately.