2) Place the potatoes on a flat baking tray with the olive oil, salt, pepper, garlic and rosemary. With clean hands, toss all the ingredients together, making sure the potatoes are covered with oil. Spread the potatoes in a single layer with one cut side down.
3) Bake the potatoes for 30 to 35 minutes, turning over to the other cut side after 20 minutes. They should be lightly browned, crisp outside and tender inside.
4) Meanwhile, lay the cod fillets on a cutting board and sprinkle both sides with salt and pepper. Cut the fillets into 4 pieces, each about 7cm.
5) In a bowl, combine the flour, baking powder, lemon zest, cayenne pepper, 1 tsp salt, and 1/2 tsp pepper. Whisk in 120ml of water and then the egg.
6) Pour the vegetable oil into a large (30cm) frying pan to around 1cm deep and heat to about 180C.
7) Dip each fillet into the batter, allowing the excess to drip back into the bowl. Place each fillet very carefully into the hot oil; don't crowd the pieces. Adjust the heat as needed to keep the oil between 180C and 200C.
8) Cook the fish on each side for 2 to 3 minutes, until lightly browned and cooked through. Remove to a plate lined with kitchen paper and keep hot. Cook the remaining pieces.
9) Sprinkle the chips and the fish with salt to taste and serve immediately.