1. In a medium bowl, combine cole slaw mix, yogurt, scallion, 1/4 tsp of the salt and the black pepper. Cover and refrigerate.
2. Place flour, egg whites and panko in separate shallow dishes. Whisk remaining 1/2 tsp salt into flour. Coat each piece of fish with flour, egg white and panko.
3. In a large nonstick skillet, heat oil over medium-high heat. Cooking in batches, saute fish 2 minutes per side, until golden. Place on a wire rack and keep warm.
4. Heat a stovetop grill pan to medium-high. Slice avocado in half; remove skin and pit. Grill 2 minutes per side. Thinly slice, then place on a plate.
5. Gently heat tortillas in microwave.
6. To assemble tacos, double up tortillas and place a few pieces of fish, some slaw and some avocado on each. Garnish with cilantro, tomatoes and onion, if using. Serve with lime wedges, hot sauce and sour cream, if using.