Step 1

In a small bowl, whisk together 4 tablespoons of the lime juice and ¼ teaspoon salt. Stir in the sliced jalapeños and set aside.

Step 2

In a food processor, combine the lime zest, the halved jalapeño, the oregano, garlic, 1 teaspoon salt and ½ teaspoon pepper. Process until the garlic is finely chopped, about 10 seconds. Add the remaining 1 tablespoon lime juice and process until smooth, another 20 seconds, scraping the bowl as needed. Transfer to a medium bowl stir in the sour cream and mayonnaise.

Step 3

In a large bowl, whisk together the cornstarch, 2 teaspoons salt and 1 teaspoon pepper. Add the fish and toss to coat. Set a wire rack in a rimmed baking sheet.

Step 4

In a 12-inch nonstick skillet over medium-high, heat 2 tablespoons of the oil until beginning to smoke. Add half of the fish in a single layer and cook without disturbing until light golden brown, 2 to 3 minutes. Carefully flip each piece and cook until light golden brown on the second sides, another 2 to 3 minutes. Transfer to the prepared wire rack and tent with foil. Repeat with the remaining 2 tablespoons oil and remaining fish.

Step 5

Smear some of the sour cream mixture onto one side of each tortilla. Divide the fish among the tortillas and top with pickled jalapeños.