Mix the crema: Stir together the sour cream, mayonnaise, 2 tablespoons of the lime juice, the sriracha if using, and 1/4 teaspoon of the salt in a small bowl. Put it in the fridge while you work on the rest.

Toss the slaw: In a large bowl, toss the shredded cabbage with 1/4 cup of the chopped cilantro, the remaining 2 tablespoons lime juice, and 1/4 teaspoon of the salt. Set it aside so the cabbage softens slightly.

Season the fish: Stir together the chili powder, cumin, garlic powder, smoked paprika, remaining 1/2 teaspoon salt, and the black pepper. Rub the mixture over both sides of the dry cod fillets.

Sear the cod: Heat the olive oil in a large skillet over medium-high heat until it shimmers. Add the fillets and cook for 3-4 minutes per side, until the outside is golden and the fish reaches 145°F, or is opaque all the way through and flakes easily with a fork.

Flake it: Move the fish to a plate and break it into large chunks with a fork.

Warm the tortillas: Wipe out the skillet and warm the tortillas over medium heat for 20-30 seconds per side, until soft and pliable. Stack them under a clean kitchen towel to keep them warm.

Build the fish tacos: Lay a spoonful of slaw in each tortilla, top with the flaked cod and a few slices of avocado, then drizzle with the crema. Finish with the remaining cilantro and serve with lime wedges.