In a large bowl, whisk together the flour, cornstarch, baking powder and a pinch of salt. With a fork, blend in the club soda until incorporated; it's OK if the batter is lumpy. Let rest at room temperature for 10 minutes.
Preheat the oven to 350°. In a small bowl, combine the mayonnaise, pickle and ketchup and season with salt. In a deep fryer or a very large saucepan, heat 1 1/2 inches of vegetable oil to 375°. Pat the flounder dry and season lightly with salt. Dip 2 of the fillets in the batter and let any excess drip off. Fry the fish, turning once, until golden brown and crisp, about 2 1/2 minutes. Drain the fish on a rack set over a baking sheet and repeat with the remaining fish fillets and batter.
Warm the tortillas in the oven for about 40 seconds, until hot and pliable. Spread the Russian dressing on the tortillas. Top with the sauerkraut, fried flounder, Jarlsberg cheese and microgreens. Fold the tortilla over the fillings and serve right away.