In a large bowl, combine sour cream and lime juice; season with salt and pepper. Transfer half the mixture to another container; set aside for serving. Toss cabbage, scallions, and minced jalapeno with remaining sour-cream mixture. Season again with salt and pepper.
In a large nonstick skillet, heat oil and remaining jalapeno half over medium-high heat; swirl to coat bottom of pan. Season fish on both sides with salt and pepper. In two batches (starting with larger pieces), cook fish until golden brown on all sides, 5 to 6 minutes. Discard jalapeno.
Meanwhile, warm tortillas according to package instructions.
To make tacos, fill tortillas with slaw, fish, and fresh cilantro leaves. Drizzle with reserved sour-cream mixture. Serve immediately.