For the sweet pepper mayo: Combine the mayonnaise, peppers, cornichon, parsley and lemon zest and juice in a medium bowl and stir to combine. Season with hot sauce to taste. Refrigerate while you make the flounder.
For the flounder: Heat 2 inches of vegetable oil in a medium Dutch oven to 375 degrees F.
Cut the flounder crosswise into pieces about as thick as 2 fingers. Whisk together the all-purpose flour, rice flour, seafood seasoning and baking powder in a large bowl (no need for salt as the Old Bay has salt in it). Whisk in the club soda to make a smooth batter slightly thinner than pancake batter.
Spread flour for dredging on a plate and season with salt and pepper. Dredge half of the flounder in the flour, then dip in the batter and add to the hot oil. Fry until golden and very crisp, turning occasionally, 4 to 5 minutes. Drain on paper towels.
Return the oil to 375 degrees F and immediately dredge, batter and fry the second batch. Serve right away with the sweet pepper mayo.