In a large bowl, combine the yogurt, mayonnaise or sour cream and the lime juice; season with salt and pepper. Transfer half the sauce mixture to another container; set aside for serving. Toss the mano, scallions, and minced jalapeño with the remaining sauce mixture. Season again with salt and pepper.
In a large nonstick skillet, heat the oil and remaining jalapeño half over medium-high heat; swirl to coat the bottom of the pan. Season the fish on both sides with salt and pepper. In two batches (starting with any larger pieces), cook the fish until golden brown on all sides, 5 to 6 minutes. Discard the jalapeño.
Meanwhile, warm the tortillas, either in another skillet on the stove, or wrapped in foil in the oven.
Fill the tortillas with slaw, fish, and fresh cilantro leaves. Drizzle with the reserved sour-cream mixture and serve immediately.