Toast the croissants. Cut the croissants in half lengthwise and lightly mist with oil. Place in the air fryer basket and toast the croissants for 3-4 minutes at 330˚F/165˚C. Remove the croissants from the air fryer basket once they are finished and tear or cut them into bite-sized pieces.
Cook the bacon. Place the bacon slices in the air fryer basket with a slice of bread beneath the basket. Cook at 380˚F/190˚C for 8-10 min. Halfway through, open the air fryer and add in the shallots or red onions. Continue cooking to bake the bacon and sauté the shallots. Once finished, remove the air fryer and let the bacon cool slightly before chopping.
Make the wet mixture. In a medium to large bowl, combine the eggs, milk, heavy cream, salt, and pepper.
Combine and rest. Layer the croissants in a lightly greased air fryer cake barrel pan. (I used a 7" pan). Add the chopped bacon and shallots, then layer on the shredded cheese. Pour the egg/milk mixture over the top and make sure it is covering all the croissants. Cover the pan with foil and let it sit in the fridge anywhere from 4-12 hours.
Cook in the air fryer. Preheat the air fryer to 400˚F/200˚C for 5 minutes. Leave the dish covered and place it in the preheated air fryer. Air fry at 350˚F/175˚C for 30-40 minutes or until internal temp reaches 185˚F/196.1˚C. Remove the foil the last 5-10 minutes to toast the top of the casserole.