Preheat the oven to 200C/400F/Gas 6.
To make the fish cakes, cook the potatoes in a large saucepan of boiling water until tender. Drain and set aside to dry for a couple of minutes.
Mash with a potato masher until smooth then mix in the butter. Taste and add salt and pepper as necessary. Set aside to cool.
Meanwhile, half fill a frying pan with boiling water and place over a medium heat. Add the defrosted fish fillets and poach gently for 5-6 minutes, or until just cooked through. Drain, pat dry with kitchen paper and leave to cool for 10 minutes.
Break the fish into flakes and add to the mashed potato. Taste and add salt and pepper as necessary.
Put the flour into a shallow bowl and season with a pinch of salt and pepper. Put the beaten egg in a separate shallow bowl and the breadcrumbs in another shallow bowl. Shape the potato and fish mixture into four large patties.
Dust a fishcake in the flour, shaking off any excess, then dip into the egg, making sure all sides are coated and finally roll in the breadcrumbs, coating all the sides in crumbs. Set to one side while you repeat with the other fishcakes. At this point the fishcakes can be chilled for up to 24 hours until needed, but otherwise refrigerate for at least 20 minutes to allow them to firm up.
Line a baking tray with baking paper and place the fishcakes on top. Drizzle with olive oil and bake for 15-20 minutes, turning after 10 minutes.
While the fishcakes are cooking, make the creamy peas. Heat a sauté or frying pan over a medium-high heat. Add the butter and, once hot, add the onion along with a pinch of salt. Fry gently until the onions are tender and translucent. Add the peas and crush lightly with the back of a fork. Once the peas are heated through, stir in the cream cheese until melted. Taste and add salt and pepper as necessary.
Serve the hot fish cakes with the creamy peas alongside.