Preheat your air fryer to 400°F (200°C) for about 5 minutes.

Set up your breading station using four shallow dishes: milk in the first, flour mixed with garlic and onion seasoning in the second, ranch dressing in the third, and panko mixed with dry ranch in the fourth.

Coat each portobello mushroom cap in the milk, then the flour mixture, ensuring even coverage.

Dip each floured mushroom into the ranch dressing, allowing excess to drip off.

Roll each coated mushroom in the panko breadcrumb mixture, pressing gently for an even crust.

Place the breaded mushrooms in a single layer in the air fryer basket and lightly spray the tops with nonstick cooking spray.

Air-fry the mushrooms for about 7 minutes, checking halfway through for even cooking.

Remove the mushrooms once golden brown and crispy. Let cool slightly before serving.