How to Make It
In a bowl, whisk the mayonnaise with the ketchup, lemon juice and Tabasco.
Pour the water into a large bowl. Using a handheld electric mixer at low speed, beat in the 2 cups of flour, 1/2 teaspoon of salt and the baking soda. Strain the batter into another bowl.
In a large saucepan, heat the oil to 360°. Line a rack with paper towels. Dust 5 pieces of cod with flour, then tap off the excess. Dip the cod in the batter, scraping the pieces lightly against the side of the bowl to remove excess batter, and add to the hot oil. Fry over moderate heat until golden and crisp, about 7 minutes. Using a slotted spoon, transfer the fish to the rack. Repeat with the remaining fish and batter. Sprinkle with salt and serve at once with the sauce.