Slice chicken breasts into 4 thin cutlets.
Set up three bowls with cornflakes, seasoned flour, and whisked egg whites with salt.
Coat each cutlet in flour, then egg whites, then cornflakes, pressing to adhere.
Spray air fryer rack with oil and place two cutlets inside, then lightly spray tops.
Air fry at 450°F for 8 to 10 minutes, flip, spray again, and cook 5 to 6 more minutes.
Repeat with remaining chicken.
Whisk sauce ingredients together, adding water as needed for consistency.
Assemble sandwiches with sauce, lettuce, chicken, pickles, and onion on buns.