Preheat your oven to 425°F. Line a baking sheet with foil, place a wire rack on top, and spray it with cooking spray to prevent sticking.
In a shallow dish, whisk the egg white until frothy. Crush the cornflakes in another shallow dish.
Season the cod pieces on all sides with garlic powder, ground pepper, and salt.
Dip each piece of cod in the whisked egg white, letting the excess drip off, then coat it in the crushed cornflakes. Press gently to ensure the flakes adhere well.
Place the coated fish pieces on the prepared wire rack. Lightly spray the tops and sides with olive oil cooking spray for a golden finish.
Bake the cod for 15-20 minutes, until the coating is crispy and an instant-read thermometer reads 145°F when inserted into the thickest part.
Meanwhile, prepare the sauce by mixing the mayonnaise and dill pickle relish in a small bowl.
Assemble the sandwiches: Place a piece of crispy cod on each whole-wheat bun, add a dollop of sauce, and layer with your favorite fresh vegetables. Serve immediately.