Slice the peppers and onions.
Melt the butter over medium heat.
Add the onions and peppers and cook, stirring occasionally until lightly browned and softened, about 7-8 minutes.
Add the salt, mustard and beer. Stir well to combine.
Turn the heat to medium low and allow to simmer until thickened. Remove from heat and keep warm until ready to serve.
Place the brats in a single layer in the air fryer basket.
Cook at 375 degrees F for 6 minutes. Flip the brats. And cook again for 6 minutes - for a total of 12 minutes.
Check the temperature of the brats with a meat thermometer. Brats are fully cooked when their internal temperature reaches 160 degrees F.
Serve the brats in buns, topped with the beer braised peppers and onions.