Place a rack in center of oven; preheat to 425°. In a large bowl, whisk oil, chili powder, garlic powder, paprika, oregano, cumin, and 1 ½ teaspoons salt. Add cauliflower and toss to coat. Arrange cauliflower cut side down on a foil-lined baking sheet.
Roast cauliflower until tender in the center and deeply browned on the bottom, 25 to 30 minutes. Let cool 5 minutes on sheet.
Meanwhile, in a small bowl, stir sour cream, 1 tablespoon lime juice, and ½ teaspoon salt. Cover and refrigerate until ready to use.
In a medium bowl, toss cabbages, onion, ¼ cup cilantro, and remaining 2 tablespoons lime juice and ½ teaspoon salt. Set aside at room temperature until ready to use.
To serve, fill each tortilla with a handful of slaw. Top with 3 to 4 pieces cauliflower and drizzle with crema. Top with more cilantro. Serve with lime wedges alongside.
