1. Preheat the oven to 200°C/400°F/gas mark 6. Bring a large pan of salted water to the boil, Peel the potatoes, cut into even-sized chunks and boil for 15 minutes or until tender.
2. Drain the potatoes well then mash until smooth. Stir in the butter, milk and cheese and stir to combine.
3. Put the remaining ingredients into a 2 litre Pyrex casserole, stir to combine and season with salt and pepper. Spoon the mashed potatoes over the top of the fish mixture then bake in the oven for 35-40 minutes or until the potato is golden and crispy on top and the fish is cooked through. Leave to cool for a few minutes before serving.