INSTRUCTIONS

1. In a large, heavy pot fitted with a candy thermometer, pour a 2-inch depth of vegetable oil. Place over medium heat until it registers 375°F.
2. Meanwhile, for the mayo, whisk together yolk with vinegar, garlic and salt in a medium bowl until combined. Using a ½ tsp measuring spoon, vigorously whisk in oil, a few drops at a time, until fully incorporated. Continue dropping in oil in ½ tsp increments, whisking constantly, until an emulsification starts and only ½ cup oil remains. Very slowly stream in remaining oil, whisking constantly, until fully combined. Mayonnaise with be thick but loose enough for dipping. Scrape into a small serving dish, cover with cling film and refrigerate until ready to serve.
3. Meanwhile, for the fish, set a wire rack on the countertop. Place 3/4 cup flour in a shallow dish or pie plate. Cut cod crosswise into 2-in wide portions. Pat dry with kitchen towel, ( then season all sides with 2 tsp salt and pepper. Working with 1 piece at a time, dredge fish in flour, turning to coat. Shake off any excess, then transfer to wire rack and set aside.4
4. Place a second wire rack over a baking sheet. Combine remaining 1½ cups flour and ½ tsp salt with baking powder in a medium bowl. Gently pour sparkling water into a large bowl, then slowly whisk flour-mixture into water until no streaks remain.
5. Working in batches of 2 or 3 pieces, dip fish into batter to fully coat, then lift with a fork, letting excess drain off. Very carefully lower into heated oil. Fry fish, turning often, until deeply golden-brown, 7 to 8 minutes. Remove with tongs or a slotted spoon to prepared rack to drain.
6. To serve, arrange fish on a large platter with potatoes, radishes and lemon wedges. Add mayonnaise dish, sprinkling with chives. Serve immediately.