Clean the mushrooms with a damp cloth and remove the stems if desired. Pat them dry.

Prepare the Mushrooms

In a bowl, combine the flour, cornstarch, salt, and black pepper. In another bowl, whisk together the egg and milk until well combined.

Make the Batter

Dip each mushroom into the egg mixture, then coat it in the flour-cornstarch mixture, pressing gently to adhere.

In a large skillet or deep fryer, heat the oil over medium-high heat until it reaches 350°F (175°C).

Carefully add the coated mushrooms to the hot oil in batches, frying for about 3-4 minutes or until golden brown. Remove and drain on paper towels.

Sprinkle the fried mushrooms with grated Parmesan cheese and serve with ranch dressing on the side for dipping.