To air-fry the frozen fish fillets, drizzle each side with the oil, then cook at 200C for 10-12 mins, turning halfway through until golden and crisp. Once the fish is cooked, top each fillet with a slice of cheese, if using, and return to the air fryer for 1 min to melt slightly. Toast the buns, then spread the bases with the tartare sauce. Add the lettuce, cheese-topped fish, finish with the bun tops and serve.