In a small bowl, whisk together mayonnaise, sour cream, garlic powder, onion powder, dill, parsley, salt, and pepper.
Add milk, one tablespoon at a time, until the dressing reaches your desired consistency. Cover and refrigerate until ready to serve.

In one bowl, mix the flour, cornmeal (if using), garlic powder, paprika, salt, and black pepper.
In a separate bowl, whisk together the eggs and milk.

Dip each mushroom slice into the egg mixture, then dredge it in the flour mixture, ensuring it’s fully coated.
Place the coated mushrooms on a plate and let them rest for 5 minutes to help the coating adhere.

Heat about 2 inches of vegetable oil in a deep skillet or pot to 350°F (175°C).
Fry the mushrooms in batches, being careful not to overcrowd the pan. Cook for 2-3 minutes, or until golden brown and crispy.
Use a slotted spoon to transfer the fried mushrooms to a plate lined with paper towels to drain excess oil.

Arrange the fried mushrooms on a platter and serve with the chilled ranch dressing for dipping.