Add the sliced onions. Bring to a low simmer. Keep the heat as low as possible so that you can see the steam coming from the pan, but with almost no bubbling. We are cooking low and slow to keep the casings from bursting. Cook for 20 minutes like this.
Remove brats from the beer. Discard the beer, but keep the onions. Place brats in air fryer basket. Cook at 350 degrees F for about 4 minutes. Add the onions over the brats. Increase the temperature to 370 degrees F and cook for another 4 to 5 minutes to brown the bratwurst. When internal temperature reaches 160 degrees F, it is safe to eat.
Serve with buns and condiments.
Notes
Store leftover cooked brats in the refrigerator for up to 5 days. I have found that reheating in the microwave is the best way to quickly reheat them without overcooking.
Freeze leftovers in freezer safe airtight containers for up to three months.