Preheat your air fryer to 390°F (195° Celsius).
Wash peppers and pat them dry.
Cut each pepper in half and remove the seeds. Then cut it into 1-inch strips or squares.
Slice onion in half, then thinly slice into half-circles.
Rinse mushrooms and pat them dry. Cut each mushroom in half, or into quarters for larger mushrooms.
In a large bowl add sliced peppers, mushrooms, onions, oil, salt & pepper to taste and season.
Toss them all together until fully coated.
Transfer veggies to the air fryer basket/tray, and lay them in a single layer.
Air fry them for 10 to 15 minutes (toss them every five minutes so that they cook evenly from each side). I did mine for 13 minutes total and the veggies came out charred with gorgeous crispy skin.
Fry them until their desired color and crispiness.
When done, the onions should be translucent. Peppers should be golden brown with slightly charred skins. Mushrooms should be dark brown in color, slightly wrinkly on the outside, and juicy on the inside.