For the fish: Drizzle each fish fillet with oil and sprinkle lightly with salt. Dust both sides of each fillet with a thin, even layer of the taco seasoning.
Heat a large cast-iron pan on the hottest part of a grill or on the stove over high heat. Add a thin layer of oil to the pan and heat until the oil just begins to smoke. Carefully add the fish and cook, undisturbed, until blackened, about 3 minutes. Gently turn the fish over with a spatula and blacken on the second side, another 3 minutes. Remove the fish to a rack to cool slightly before plating.
For the slaw: Combine the cabbage, carrots and scallion in a large bowl. Season with salt and pepper and mix well. Set aside.
In a separate bowl, whisk together the jalapeno, mayonnaise, sour cream, vinegar, sugar and chili seasoning in a medium bowl. Add to the cabbage mixture and mix well to combine. Taste for seasoning and add more salt, pepper or sugar if desired.
Place a few strips of the blackened fish on a warmed soft tortilla and drizzle with Charred Scallion Salsa. Top each taco with slaw and serve with a wedge of lime.
Charred Scallion Salsa:
Preheat a grill for cooking over medium-high heat. Place the scallions and jalapeno on a large plate or baking pan. Drizzle with oil and sprinkle with salt and pepper.
Place the vegetables on the grill and cook until charred on all sides, about 10 minutes total.
Combine the cilantro, scallions and jalapeno in a blender or food processor and pulse until finely chopped. Transfer to a bowl and stir in the paprika, garlic, cumin, lime juice and olive oil. Season to taste with salt and pepper. Store tightly covered in the refrigerator up to 10 days.