Stir the sour cream, mayonnaise and 1 tablespoon of the lime juice together with a pinch of the salt. Refrigerate until serving.
Toss the shredded cabbage with the cilantro and the remaining 2 tablespoons of lime juice no more than 15 minutes before you eat, so it stays crisp.
Pat the fish completely dry on both sides, then season it with the chili powder, cumin and remaining salt.
Heat the oil in a heavy skillet over medium-high until it shimmers. Lay the fish in and leave it undisturbed for 3 to 4 minutes, until the underside is browned and releases from the pan on its own.
Turn once and cook 2 to 3 minutes more, until the flesh is opaque and flakes when pressed with a fork. Do not stir or break it up.
Warm the tortillas on a dry pan for about 20 seconds a side, until soft and flexible.
{'Break the fish into large pieces and build each taco': 'fish first, slaw on top, then a drizzle of the crema. Serve with extra lime.'}