Wash the potatoes and put them onto a metal baking sheet. Scatter a little water round the potatoes (just use your hands). Pierce each potato with a fork and sprinkle sea salt on top of them. Put into an oven preheated to 200°C and cook for 45 minutes – 1 hour. The potatoes should be tender through to the middle.
While the potatoes are cooking melt half the butter in a frying pan and saute the onions until soft and pale gold. Scrape these into a bowl.
Melt the rest of the butter and cook the fish on both sides until pale gold, then turn the heat down low, cover the pan and let it cook gently until it is cooked through. It will only take about 4 minutes.
Remove the skin from the fish (if it has any, not all fillets come with the skin on). Flake the flesh and add it to the onions along with any buttery juices that are in the frying pan.
Halve the potatoes, scoop out the potato flesh inside using a spoon and add it to the onions and fish. Don’t scoop out so much that the potato skin collapses – it has to keep its shape.
Mash everything together, adding the seasoning and parsley. Now pile the mash back into the potato cakes.
Sprinkle the cheese over the top and return to the oven for 15 minutes. The cheese should be melted and golden. Serve immediately.