Heat oven to 220C/200C fan/gas 7. Sit the potato on a large spoon and cut slits across it every 0.5cm (the edges of the spoon will stop the knife cutting all the way through the potato). Microwave for 10-12 mins until starting to soften (alternatively, bake in the oven for 1 hr 10 mins).
Meanwhile, prepare the topping for the fish. Blitz the bread in a food processor into breadcrumbs (or grate it). Combine the breadcrumbs with the orange zest, dill, garlic and seasoning. Season the cod, put it on a baking tray and top with the crumb.
Put the potato on the same tray, drizzle with oil and sprinkle with salt flakes. Bake for 12-15 mins until the fish is cooked and the potato is tender but crispy. Serve with cr ème fraîche and steamed vegetables, if you like.