Brussels Sprouts: Remove ends and any leaves that have seen better days. Slice Brussel sprouts in half. Each should be about 1/2 inch thick. If Brussels sprouts are large (larger than an inch or so), you may want to quarter them for quicker cooking time. Try to ensure they're of uniform size.

Onions: Trim ends and remove skin. Slice each in half to create half-moons. Slice each half moon into pieces about 1/4-1/2 inch thick.

Toss prepared Brussels sprouts and onions in a medium bowl with olive oil, salt and pepper.

Cook in an air fryer at 400F for 20 minutes, mixing halfway through, until crispy and cooked throughout. Test, and if needed, continue cooking at 5 minute intervals, until Brussels sprouts reach desired doneness.