Step 1: Prepare the Fish

1. Pat the fish fillets dry with paper towels, then cut them into bite-sized pieces.
2. In a shallow dish, mix the flour, garlic powder, onion powder, paprika, salt, and black pepper.
3. Pour the buttermilk into a second bowl.
4. In a third dish, combine the panko breadcrumbs and cornmeal.
5. Dip each fish piece into the flour mixture, then the buttermilk, and finally coat it in the breadcrumb mixture.
6. Heat about ½ inch of vegetable oil in a large skillet over medium-high heat.
7. Fry the fish in batches for about 2-3 minutes per side until golden brown and crispy. Transfer to a plate lined with paper towels to drain excess oil.

Step 2: Make the Slaw

1. In a large bowl, combine the shredded cabbage, carrots, and cilantro.
2. Squeeze in the lime juice, then add salt and pepper. Toss to mix well.

Step 3: Prepare the Sauce

1. In a small bowl, whisk together the mayonnaise, sour cream, lime juice, hot sauce (if using), garlic powder, and cumin.
2. Set aside for later assembly.

Step 4: Assemble the Casserole

1. Preheat the oven to 375°F (190°C).
2. Lightly grease a 9×13-inch baking dish.
3. Spread half of the torn corn tortillas evenly over the bottom of the dish.
4. Add half of the crispy fish pieces on top.
5. Sprinkle with half of the shredded cheese.
6. Drizzle half of the prepared sauce over the layers.
7. Repeat the layers with the remaining tortillas, fish, cheese, and sauce.

Step 5: Bake the Casserole

1. Cover the casserole with foil and bake for 20 minutes.
2. Remove the foil and bake for an additional 10 minutes until the cheese is melted and bubbly.

Step 6: Serve and Garnish

1. Remove the casserole from the oven and let it cool for 5 minutes.
2. Top with the cabbage slaw, diced tomatoes, and green onions before serving.