Clean the mushrooms with a damp paper towel and trim any long stems. Pat them completely dry so the coating adheres well.
In a large bowl, whisk together the flour, cornstarch, baking powder, garlic powder, onion powder, paprika, oregano, salt, and black pepper. Gradually whisk in the cold water until a smooth batter forms.
Place the panko breadcrumbs in a separate shallow bowl.
Heat about 3 inches of oil in a deep pot to 350°F (175°C).
Dip each mushroom into the batter, allowing the excess to drip off, then roll it in the panko breadcrumbs until evenly coated. Arrange the coated mushrooms on a tray while preparing the remaining batch.
Working in batches, carefully lower the mushrooms into the hot oil. Fry for 2 to 3 minutes, turning occasionally, until they are deep golden brown and crispy.
Transfer the fried mushrooms to a wire rack or paper towel-lined baking sheet to drain briefly.
Garnish with chopped parsley and serve immediately with your favorite dipping sauces while they're still hot and crisp.