Make the Lime Garlic Crema: Combine the sour cream and garlic in a small bowl. Grate the zest of one lime into the mixture and add the juice from the lime. Add salt to taste, stir well, and set aside.
Prepare the Red Cabbage Slaw: Toss together the red cabbage, scallions, and minced jalapeños in a large mixing bowl. Add 2 teaspoons of lime juice and the olive oil. Season with ½ teaspoon of salt and black pepper, stirring to coat the vegetables evenly. Set this aside for later use.
Season and Broil the Fish: Move an oven rack to about 4 to 6 inches below the broiler and preheat the broiler to high. In a small bowl, mix together the cumin, chili powder, and ¾ teaspoon of salt. Rub this spice blend over the fish, ensuring it's evenly coated. Lightly drizzle the fish with olive oil, then place it on a rimmed baking sheet. Broil the fish for 3 to 4 minutes per side, flipping halfway, until cooked through and flaky.
Warm the Tortillas: Heat the tortillas briefly, either directly over an open flame until slightly charred or in a hot skillet until softened.
Assemble the Tacos: Use a fork to break the broiled fish into small chunks. Place the fish on the warmed tortillas, then top with a generous serving of the red cabbage slaw. Drizzle with the lime garlic crema and, if desired, add slices of avocado. Garnish with fresh cilantro leaves and serve with lime wedges on the side for an extra citrus kick.