Preheat your air fryer to 350ºF (177ºC) for 4 minutes.
Clean and trim the Brussels sprouts. Using a sharp knife and a cutting board, carefully cut the base of the Brussels sprouts and remove any outer leaves with discoloration. Then, give it a quick rinse and set it aside for air frying.
Air Fry the Brussels sprouts at 350ºF (176ºC) for 10 - 12 minutes. Instead of boiling, we'll soften the Brussels sprouts by air frying them at low heat. You can also boil them for a softer mushy core if you want.
Once the Brussels sprouts become tender, transfer them to a large bowl. Add olive oil, salt, pepper, garlic, parmesan cheese, and chili flakes. Toss everything together until the Brussels sprouts are evenly coated in the seasoning mixture.
After coating the Brussels sprouts with seasoning, transfer them to a flat surface. Then, smash the Brussels sprouts with a glass, mug, or a small bowl with a flat base. I tried using a potato masher, but the Brussels sprouts are still too hard. You can add more seasoning at this stage.
Transfer smashed Brussels sprouts to the air fryer, ensuring they don't overlap. Air fry the Brussels Sprouts at 400ºF (204ºC) for 5 - 8 minutes until they are golden brown and crispy.
Serve and enjoy your crispy air fried smashed Brussels sprouts, garnished with additional chili flakes and parmesan cheese if desired.