Heat the oven to 400°. Line a baking sheet with foil, then coat it with cooking spray.
In a wide, shallow bowl, whisk together the mayonnaise, mustard, and ketchup. Place the bread crumbs in a second bowl.
Cut the fish crosswise into pieces roughly 1 inch thick. Thin sections can be folded to create similar-size fingers. One at a time, dredge the fish pieces in the mayonnaise mixture. Shake off any excess (the coating will be thick). Roll them in the bread crumbs to coat evenly.
Arrange the fish on the prepared baking sheet, then bake for 15 to 20 minutes or until the coating is lightly browned and the fish flakes easily.