To prepare plank, immerse and soak plank in water 1 hour; drain.
Prepare grill, heating one side to medium and one side to high heat.
To prepare slaw, combine 1/2 teaspoon salt, shredded cabbage, salad greens, onions, sour cream, and lime juice; toss well to coat. Chill.
To prepare tacos, combine chili powder, black pepper, and 1/4 teaspoon salt in a small bowl. Sprinkle chili powder mixture over fish. Place plank on grill rack over high heat; grill 5 minutes or until lightly charred. Carefully turn plank over; move to medium heat. Place fish on charred side of plank. Cover and grill 18 minutes or until fish flakes easily when tested with a fork.
Warm tortillas according to package directions. Break fish into chunks, and place about 2 ounces fish on each tortilla. Top each taco with 1/4 cup slaw; fold tortillas in half.