Prep the broccoli. Rinse and dry thoroughly.
Cut the head into medium florets, about 1.5-inch pieces. Slice the thick stalk into coins or batons so nothing goes to waste.
Dry it well. Moisture prevents crisping. Pat the florets dry with a clean towel.
If they’re damp, they’ll steam instead of roast.
Season. In a large bowl, toss broccoli with olive oil, garlic, salt, pepper, and red pepper flakes. Add lemon zest if you’re using it. Everything should be lightly coated, not drenched.
Preheat the air fryer to 390–400°F (200–205°C) for 3–5 minutes.
Lightly spray the basket if sticking is a concern.
Arrange in a single layer. Don’t overcrowd. If needed, cook in two batches. A bit of space around each floret helps the edges crisp.
Air fry for 6–10 minutes. Shake the basket or toss halfway.
Cook until the tips are browned and the stems are crisp-tender. Thinner florets cook faster; keep an eye on them.
Finish with acid and cheese. Transfer to a bowl. Squeeze over lemon juice and toss.
Sprinkle with Parmesan if you like. Taste and adjust salt.
Serve immediately. Broccoli is best hot, when the edges are still crackly and the centers are tender.