How to Make It
Preheat the air fryer to 400°F for five minutes.
Grease the wire basket with cooking spray. Add the quartered onions to the basket, spray gently with cooking oil, and toss to coat evenly. Cook for 10 minutes, shaking halfway through.
Mix butter, chives, black garlic, and salt together until combined. Set aside.
Season the steak with salt and pepper and brush the top with a small amount of the butter mixture.
When the onions are done, place the steaks into the prepared basket, spacing them out with enough room for air to circulate. Cook until desired doneness, flipping halfway through. For medium rare, cook for 10 minutes; for medium, 12 minutes; for medium well, 14 minutes.
Place the steaks on a cutting board and let rest for five minutes. Slice across the grain, top with the remaining butter, and serve alongside the crispy onions.