Mix egg and water and slowly whisk in flour and cornmeal. Add parsley, garlic salt, onion salt, salt, and ground white pepper. Lay prepared fish in batter while oil is heating.
In heavy skillet (preferably cast iron or non-stick), heat oil over medium-high heat until a 1/2 teaspoon batter dropped into it sizzles and begins to brown. Fry fish for 3 minutes on each side. Remove and drain on paper towels.
Garnish with lemon slices and flat-leaf parsley leaves.