Preheat your air fryer to 400ºF or oven to 450℉
Pat the flounder fillets dry with paper towels. Arrange them on a parchment liner sized to fit your air fryer basket or sheet pan. Spread the mayonnaise evenly over the top of each fillet, then sprinkle evenly with Old Bay seasoning.
Air fry or bake the flounder until golden brown and the flesh flakes easily with a fork, about 10 minutes (8 minutes for very thin fillets; up to 12–15 minutes for thick ones). If using the broiler as an alternate cooking method, place the fish on a lightly oiled, foil-lined sheet pan to prevent sticking (skip parchment—it can burn under the broiler).
Serve hot with lemon wedges for squeezing over. Heaven!