Prep: Wash Brussels sprouts and pat thoroughly dry with paper towels. Trim the stem end and halve lengthwise. Quarter any large sprouts. Remove and discard loose outer leaves.
Toss: In a large bowl, toss Brussels sprouts with olive oil, garlic, salt, pepper, and red pepper flakes (if using). Make sure every cut surface is coated.
Air fry: Arrange Brussels sprouts in a single layer in the air fryer basket, cut side down where possible. Do not overcrowd — cook in batches if needed.
Cook at 380°F (195°C) for 12–14 minutes, shaking the basket halfway through (at the 6-minute mark). Sprouts are done when the cut surfaces are deep golden brown and the edges have charred slightly.
Finish: Transfer to a serving bowl. Drizzle balsamic glaze over the hot sprouts and toss lightly. Finish with flaky salt.
Serve immediately.