Preheat the air fryer to 400°F. Spritz the air fryer basket with olive oil spray.
In a small bowl, stir together 3 tablespoons mayonnaise, the minced 6 basil leaves, and 2 tablespoons of the grated Parmesan.
Pat the 4 salmon fillets dry. Squeeze the juice from the 1/2 lemon over the fillets, then season evenly with 1/4 teaspoon Kosher salt and freshly ground black pepper to taste.
Spread the mayonnaise–basil–Parmesan mixture evenly over the top of each fillet. Sprinkle the remaining 1 tablespoon grated Parmesan over the tops.
Arrange fillets in a single layer in the air fryer basket (work in batches if they do not fit without overlapping). Air fry at 400°F for about 7 minutes, or longer depending on thickness, until the salmon is opaque throughout and flakes easily with a fork (or reaches 145°F internal temperature).
Carefully remove the fillets from the air fryer, garnish with additional basil leaves if desired, and serve.