Prep the Onion: Peel the onion and slice off both ends. Cut out the center (core), leaving a well for the butter, but do not cut all the way through the bottom.
Season and Stuff: Place the onion on a sheet of aluminum foil. Stuff the center with 1–2 tablespoons of butter and sprinkle with half of the seasonings.
Add Extra Butter: Place additional chunks of butter around the outside of the onion and top with the remaining seasoning.
Wrap: Carefully wrap the onion tightly in the foil to create a packet.
Air Fry: Place the packet in the air fryer basket. Air fry at 385°F (195°C) for 45 to 50 minutes until very tender.
Serve: Let it rest for a few minutes, as it will be very hot. Open carefully and enjoy.