Slice the onion in ¼" slices. Put the onion slices in the inner pot and spritz with oil. Place the rack into the Ninja Foodi in the high position and turn the Broil on for 10 minutes. This will start to cook the onions and preheat the Foodi for the burgers.
Divide the ground beef into 2 ounce portions and form 4 burgers of equal size. Put a thumbprint in the center of each burger to prevent puffing during the cooking process. Place the burgers on the rack in the high position. Set the broil for 10 minutes. Filp at 5 minutes.
Remove the burger patties and drain on a paper towel. Remove onions and place in small bowl.
Butter one side of each slice of bread with 1 tsp of butter. Add 1 tsp of mayo to unbuttered side and place two pieces of cheese on each slice of bread. Put burger patties on the sandwich and top with onions. Close up the sandwich. Sprinkle garlic bread seasoning on the buttered side of the sandwich if desired.
Remove the rack from Ninja Foodi and turn to the low position. Place sandwich on the rack. Return to the Ninja Foodi and set the broil function on 10 minutes. At about 5 minutes, remove the rack and carefully flip the sandwich. Return to the Foodi and continue broiling for 3-5 minutes until desired browning has occurred. You can flip it back over if you want to brown the other side a little more.