Pat the shrimp dry: Lay the shrimp on a couple layers of paper towels, then press another layer on top. Press firmly to absorb all moisture until the surface looks matte. This is the step that makes the coating stick and the edges crisp.
Mix the coating: In a medium bowl, stir together the parmesan, garlic powder, smoked paprika, salt, pepper, and red pepper flakes if using. Break up any clumps with a fork.
Toss the shrimp: Add the shrimp to the bowl, drizzle with olive oil, and toss until every shrimp is coated with a thin, speckled parmesan crust. If it looks dry, add a tiny splash more oil.
Preheat the air fryer: Set it to 400°F and let it run for 3 minutes. A hot start makes the difference between crispy and floppy.
Arrange in a single layer: Place the shrimp in the air fryer basket in a single layer without overlapping. Overcrowding steams them; do two batches if needed.
Air fry: Cook at 400°F for 6 to 8 minutes. Gently shake the basket at the 3-minute mark. Look for golden edges and a pink, opaque center.
Check for doneness: The shrimp should curl into a soft C shape and the coating should look golden. If using a thermometer, 145°F is your number. If they look done at 6 minutes, take them out.
Let them rest: Give them 2 minutes out of the air fryer. The coating sets as it rests.
Serve: Toss with fresh parsley, squeeze lemon over the top, and serve hot.