Add the green beans, mushroom soup, diced onions and chopped bacon to a small casserole dish (that will fit in the basket of your air fryer).
Mix together well then top with first the grated cheddar then the crispy fried onion.
Once the air fryer is ready add the dish to the basket and cook for 15 minutes.
Notes
If you want to keep the green bean casserole warm in the air fryer, you may want to open the basket first to let some of that heat out. You don't want to burn the onions on top.