Grease ramekin with cooking spray. Preheat Air Fryer for 3 minutes at 350 F.
Arrange bacon slice so that it's folded into ramekin. Add chopped onion and cook for 5 minutes.
In bowl, crack egg and whisk. Add milk, salt and pepper and mix.
Remove hot ramekin from Air Fryer and add egg mixture.
Place ramekin back onto Air Fryer tray. Cook for 2 minutes. Poke the middle so that it will cook all the way through. It should be runny in the middle and crusty on the outside.
Sprinkle cheese on top. Cook for another 3 minutes, or until golden and melted.
Carefully remove from Air Fryer. It will be really hot, so use tongs, a towel or gloves. Add garnishes as desired.