1. Place racks in top and middle of oven; preheat to 350ºF. Spread panko on a rimmed baking sheet and bake until golden, stirring once, about 15 minutes. Transfer to a bowl. Raise oven temperature to 450ºF. Line a baking sheet with foil, then mist with cooking spray.
2. In a bowl, whisk together mayonnaise, eggs and salt until smooth. Place flour and bread crumbs in separate bowls. Coat fish fingers in flour, dip in egg mixture and coat with panko, shaking off excess at each step. Place fish pieces in a single layer on lined baking sheet as finished. Mist with cooking spray.
3. Bake on top rack until fish is just cooked through, 6 to 10 minutes. Serve immediately.