Trim and halve the brussel sprouts and place into a bowl with olive oil and salt, and toss to cover the sprouts evenly.
Select Air Fry and set the temperature to 190°C and the time to 13 minutes.
When prompted to ‘Add food,’ place the brussel sprouts in the basket in a single layer. Give them a shake at halfway.
When the cooking program finishes, add the minced onion and butter to the basket, and select Air Fry again for 2 minutes to melt the butter.
When the program finishes again, give everything a shake to move the melted butter and onion in and around the sprouts before serving.