For the mayonnaise: Whisk egg yolks in a me-dium bowl until pale yellow. Gradually add oil in a thin, steady stream, whisking constantly until mixture is emulsified and oil is completely incorporated. Whisk in lemon juice. Mayonnaise should be loose; thin with a little water, if necessary. Season to taste with salt. Transfer mayonnaise to a cream whipper (see facing page). Aerate according to manufacturer's directions into the top of a double boiler set over barely simmering water over low heat. Gently heat mayonnaise until warm (do not let it get warmer than 140°, or it will curdle).
For the fish: Pour oil into a medium pot to a depth of 2" and heat over medium heat until temperature reaches 350° on a candy thermometer. Meanwhile, whisk flour, 2 pinches salt, and eggs together in a medium bowl until batter is smooth.
Working in batches, season fish with salt, dip into batter, and deep-fry until golden and crisp, about 45 seconds per batch. Drain on a wire rack set over paper towels and season to taste with salt. Serve hot with warm mayonnaise.