Season the flour on a plate, then toss the cod loins in the seasoned flour. Place the beaten egg on a second plate, add the fish and gently coat. Toss the breadcrumbs with the parsley and lemon rind, then coat the fish in the breadcrumbs.
Heat the oil in a large frying pan, add the coated fish and cook over a medium heat for 2–3 minutes on each side, or until golden and cooked through.
Meanwhile, mix together all the tartare sauce ingredients in a small bowl.
To assemble the burgers, cut the rolls in half. Place 1 tomato slice on each of the roll bases, add the hot fish loins and top with spoonfuls of the tartare sauce and the watercress. Top with the remaining bun halves and serve.