1. Arrange two racks in top and bottom thirds of oven and preheat to 450ºF. Line two rimmed baking sheets with heavy-duty foil. Mist one sheet with cooking spray. Top second sheet with a wire rack and mist rack with cooking spray.
2. Arrange potatoes on first baking sheet, coat with a layer of cooking spray and sprinkle with salt. Bake in bottom third of oven until undersides are golden, 15 to 20 minutes.
3. While potatoes are baking, sprinkle both sides of fish with salt, dredge in flour and dip into eggs, letting excess drip off. Coat fish with crushed cornflakes. Place on wire rack on second baking sheet. Mist fish with cooking spray.
4. Turn potatoes and return to bottom third of oven. Place fish in top third of oven. Bake until potatoes are golden on both sides and fish is cooked through, about 15 minutes longer. Serve immediately.