Season halibut fillet evenly with ¾ teaspoon of the salt and ¼ teaspoon of the black pepper.
Heat 2 tablespoons of the oil in a cast-iron skillet over high.
Add fillet; cook, undisturbed, until deeply browned and a thermometer inserted in thickest portion of fillet registers 140°F, about 6 minutes per side.
Remove fillet from skillet; let rest 5 to 10 minutes.
Flake into 1-inch pieces.
While halibut rests, stir together lime juice, mayonnaise and remaining 1 tablespoon oil, ¼ teaspoon salt and ¼ teaspoon black pepper in a medium bowl.
Add coleslaw, bell pepper, cilantro and scallions; toss to fully coat.
Spoon about ¼ cup slaw mixture onto each of 8 warm tortillas; top evenly with flaked halibut.
Top each taco with ½ tablespoon pico de gallo and 1 teaspoon queso fresco.
