Preheat the oven to 450 degrees F (230 degrees C). Coat a baking dish with cooking spray.

Combine crackers, onions, bread crumbs, sesame seeds, salt, and black pepper in a bowl; set aside.

Wash and pat dry chicken breasts. Spread a thin layer of mayonnaise on one side of each breast; press into cracker mixture, mayonnaise-side down. Spread a thin layer of mayonnaise on other side of each breast; cover with cracker mixture, patting firmly until it adheres. Place coated chicken breasts into the prepared baking dish. Sprinkle remaining cracker mixture on top; lightly coat chicken with cooking spray.

Bake in the preheated oven until breasts are no longer pink in centers and the juices run clear, 35 to 40 minutes. An instant-read thermometer inserted into centers should read at least 165 degrees F (74 degrees C).