Stir the flour, cornstarch, water, and salt together in a mixing bowl. Let stand 20 minutes.
Heat several inches of oil to 375°F in a deep-fryer.
Dip the fish in the batter; drain off any excess. Fry the fish in the hot oil until crisp and golden, 1 to 2 minutes. Drain on paper towels. Serve on a white cloth napkin with lots of lemon.