Heat oil in the deep fryer to 375 degrees. In a small bowl, combine mayonnaise, sweet pickle relish, and minced onion. Stir in lemon juice. Season with salt and pepper to taste. Cover and refrigerate for at least 1 hour before serving.
In a shallow bowl or pie plate combine cornmeal, 1 cup of flour, salt, black pepper, onion powder, garlic powder, paprika, parsley flakes, dried oregano, and cayenne pepper. In another bowl beat together the egg and milk.
Pat dry the fish fillets with paper towels. Put a ½ cup of the flour mixture into a plastic bag. Add the fish fillets to the bag and shake to coat the filets with flour. Dip the filets, one by one, into the egg mixture, and then dip into the cornmeal, flour, and spice mixture.
Heat oil in a deep fryer to 375 degrees. Add the filets to the oil and deep fry in batches for about 2-3 minutes until golden brown and fully cooked (the internal temperature has reached 145 degrees). Drain on paper towels. Serve with tartar sauce.