Preheat your oven to 400F. Drizzle the frozen cod filets with olive oil and once the oven is preheated, roast for about 15 minutes, depending on the thickness of the fish.
Remove from oven and let cool completely.
Once cooled, break up the fish with a fork and add celery, red onion, lemon zest & juice, and season with salt and pepper.
Fold in mayonnaise and stuff into pita pockets layered with lettuce.