Preparation

1. In a small bowl, whisk together the mayonnaise, sour cream, milk, and Sriracha until smooth. Season to taste with salt. Refrigerate until ready to use (the crema can be made up to 2 days ahead). 
2. Heat the oil in a HexClad 3.3QT Hybrid Deep Sauté Pan until it reaches 350°. Place a wire rack inside a large rimmed baking sheet. While the oil is heating, scatter the pickled jalapeños on a paper towel–lined plate and pat dry. Cut the fish into pieces about 3 inches long by ¾ inch. In a medium bowl, whisk together the flour, baking powder, and 1 tablespoon salt. Add the seltzer and whisk until blended. 
3. When the oil is ready (if you don’t have a thermometer, drizzle in a few drops of batter to see if it sizzles), place the jalapeños in the batter and stir to coat. Using a fork or tongs, remove a few jalapeños and let the excess batter drip back into the bowl. Fry the jalapeños until browned and crisp, about 2 minutes. Transfer to the wire rack and season with salt. Repeat with the remaining jalapeños.
4. Working in batches, dip the fish in the batter and turn to coat. Using tongs, remove the fish and let the excess batter drip into the bowl. Slowly lower the fish into the oil. Fry the fish a few pieces at a time, turning frequently, until golden brown and crisp, 3 to 4 minutes. Transfer to the wire rack and season with salt. Return the oil to 350° between batches. If not serving immediately, you can re-crisp the fish by dipping it in the hot oil for about 30 seconds. 
5. Place a piece of fish inside each taco and top with avocado, cabbage, pickled onions, and cilantro. Drizzle with the baja crema and serve with lime wedges.