Make the slaw: Place the shredded cabbage in a medium bowl and toss with 1/2 teaspoon kosher salt. Add the red onion, cilantro, jalapeño, lime juice, and olive oil.
Toss the slaw well, then taste and adjust with more lime juice, salt, or jalapeño as needed. Chill while preparing the fish.
Make the sauce: In a small bowl, stir together mayonnaise or crema, chipotle pepper puree, lime juice, and a pinch of salt. Refrigerate until ready to assemble.
Preheat the cooking surface. Heat a grill to medium-high, preheat the oven to 400°F, or warm a skillet over medium-high heat with olive oil.
Pat the fish dry. In a small bowl, combine cumin, paprika, garlic powder, onion powder, kosher salt, and black pepper. Lightly season all sides of the fish with the spice rub.
To grill, oil the grates well and cook the fish for a few minutes per side, until grill marks appear and the fish is opaque and flaky. Squeeze fresh lime juice over the cooked fish.
To bake, place the seasoned fish on a parchment-lined baking sheet, lightly brush or spray with olive oil, and bake for 6 minutes. Flip, lightly oil again, and bake for 4 to 6 more minutes, until cooked through. Squeeze fresh lime juice over the fish.
To pan-sear, heat olive oil in a skillet. Sear the fish for 3 to 4 minutes on one side, flip, reduce the heat slightly, and finish cooking until the fish is flaky and opaque. Squeeze fresh lime juice over the fish.
Warm the tortillas briefly on the grill, in a dry skillet, or over a gas flame until pliable and lightly charred. Wrap them in a clean towel to keep warm.
Assemble the tacos by adding pieces of cooked fish to warm tortillas. Top with cabbage slaw and drizzle with chipotle-style sauce.
Add optional garnishes such as avocado, extra cilantro, Cotija cheese, pickled red onions, and lime wedges. Serve immediately.