Clean portobello mushroom caps with cool water and gently pat them dry with a paper towel.

In a medium bowl, whisk together the milk and ranch dressing until the mixture is smooth and well combined.

In another bowl, combine the all-purpose flour with garlic and onion seasoning.

In yet another bowl, mix panko bread crumbs with the dry ranch dressing mix.

Dip each mushroom into the milk mixture, allowing any excess to drip off, then coat it with the flour mixture.

Return the mushroom to the milk mixture, and then coat it in the panko mixture, pressing gently to help the crumbs adhere well.

Carefully place the coated mushrooms on a plate, ensuring they are spaced out and not touching.

Follow the manufacturer's instructions to preheat your air fryer to ensure that it’s hot and ready.

Lightly spray the air fryer basket with nonstick cooking spray.

Arrange the mushrooms in a single layer in the basket. Cook for about 10-12 minutes, flipping halfway through until they are golden and crispy.