Boil potatoes for 10-15 minutes until fork-tender. Drain and mash the potatoes and a small splash of the milk with a fork. Season with salt and pepper. Set aside.

Preheat the oven to 400°F (200°C).

Whisk cornstarch in with the remaining milk until combined. Add milk mixture, and butter into a pan over medium heat. Bring to a boil, stirring continuously until the sauce bubbles and thickens. Add peas and season with salt and pepper. Cook 1 minute longer then turn off heat.

Place chunks of fish in a small baking dish. Pour sauce over. Spoon mashed potatoes on top to make an even layer. Sprinkle cheese on top. Bake 25-30 minutes until top is golden. Serve.