Preheat the oven to 190°C/gas mark 5. Bring a pan of water to the boil, then add the potatoes and cook for 15 minutes until tender.
Meanwhile, put the flour on a plate, beat the egg in a shallow bowl and crush the breadcrumbs and cornflakes together with your hands on a separate plate. Dip the coley strips into the flour, shake off the excess then dip into the egg followed by the breadcrumb and cornflake mixture. Turn until evenly coated, then place on a baking tray. Drizzle with the oil and bake in the oven for 15 minutes until the coating is golden and crisp and the fish is opaque and cooked through.
When the potatoes are cooked, drain and return to the pan. Mash with the crème fraîche until soft and creamy and season with freshly ground black pepper.
Meanwhile, cook the peas in a pan of boiling water for 3 minutes. Drain the peas and mash with a potato masher or with a stick blender until crushed.
Serve the coley fingers with the mash and crushed peas with some ketchup on the side.