This recipe for battered cod is courtesy of chef Cathal Armstrong.
Place 1 3/4 cups water in the bowl of an electric mixer fitted with the whisk attachment. Turn mixer on low speed and beat in flour, 1/2 teaspoon salt, and baking soda. Strain through a fine mesh sieve set over a large bowl.
Heat oil in a large saucepan over medium-high heat until it reaches 360 degrees on a deep-fry thermometer. Place 1 cup flour in a shallow dish. Working with one piece of cod at a time, dredge cod in flour and then place in batter to coat, shaking off any excess. Carefully place in heated oil. Fry until golden and crisp, about 7 minutes. Transfer cod to a wire rack and sprinkle with salt. Repeat process with remaining fish and batter. Serve immediately with desired sauces.