Wash your peppers, then cut the tops off of your peppers, and remove the seeds & excess pizz. Discard the tops, seeds, and any pulp.
Add the prepared peppers to a large pot, and then add water just until it covers the vegetables. Bring the pot to a high simmer, and let the peppers cook for 2-3 minutes- just until slightly softened.
Carefully remove the pot from heat. Using a pair of tongs CAREFULLY transfer the hot peppers to a strainer in your sink, and run them under cold water to stop the cooking process. Once cool to the touch, drain the peppers and set them aside.
Set a large skillet over medium heat and add the beef & onions to it. Cook the beef, crumbling as it cooks, until cooked through with no pink remaining. Strain the meat to remove grease, and then return the meat mix to the skillet.
Add the remaining ingredients (EXCEPT marinara sauce & cheese) to the skillet and stir everything together until evenly combined. Spoon the mixture into the peppers, dividing it evenly out between them all.
Top each stuffed pepper with with 1 tablespoon of marinara sauce, and using the back of a small spoon spread it out slightly to cover the mixture. Repeat until all the peppers are stuffed.
Cooking in batches, at 2-3 peppers to the basket of your air fryer. Don't overcrowd- this will depend on the size of your machine. Cook them at 350° for 10 minutes.
Top each pepper with some of the shredded cheese, and cook another 3-5 minutes. Remove the cooked peppers to a waiting platter or baking dish, and repeat until all the peppers are finished.