Scrub the potatoes and cut into 1 cm (½ inch) thick slices. Cut across into chunky chips. Place in a bowl and drizzle with the oil, then turn in the oil until coated. Sprinkle with the paprika, celery salt and cumin and mix.
Heat a large roasting tin in a preheated oven, 200°C (400°F), Gas Mark 6, for 3 minutes. Tip in the potatoes and spread out in an even layer. Bake in the oven for about 40 minutes, turning the potatoes a couple of times, until evenly pale golden.
Season the pieces of fish with salt and pepper. Slide the potatoes to one side of the roasting tin and add the fish. Dot with the butter and return the roasting tin to the oven for a further 20 minutes or until the fish is cooked through. Transfer the fish and chips to warmed serving plates and return to the oven, with the door left open, to keep warm.
Add the peas and stock to the roasting tin and bring to the boil on the hob. Boil for 3 minutes or until the peas are tender. Tip the contents of the roasting tin into a food processor, add the mint and crème fraîche and process to a purée. Serve with the fish and chips.