Cut the potatoes into wedges (you should get about 8–12 thick wedges from each potato). Toss with the oil and salt and pepper to taste in a roasting tin and roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 35–40 minutes, turning once, until evenly browned.
Meanwhile, combine the breadcrumbs, polenta, thyme and salt and pepper to taste in a large bowl. Dust each fish fillet with seasoned flour and then dip into the beaten egg and finally into the breadcrumb mixture to completely coat the fish.
About 10 minutes before the chips are ready, heat 1 cm (½ inch) oil in a large frying pan, add the fish fillets, in 2 batches, and cook over a medium heat for 2–3 minutes on each side until the coating is crisp and golden and the fish is cooked through. Remove from the pan with a fish slice and keep warm in the bottom of the oven while you cook the remainder. Serve with the oven chips and tomato ketchup.