Whisk an egg in a small bowl. Set aside.
Stir together flour, panko, chili powder, salt, and pepper in a shallow bowl. Set aside.
Season the fish with salt and pepper. Using tongs, a fork, or your fingers, dip fish into the egg wash and then coat with the flour mixture.
Heat the oil in a cast iron (or non-stick) pan over medium heat until the oil is shimmering. Add the fish to the pan without crowding the fish. Cook the fish for about 3 to 5 minutes until it's cooked halfway through the sides. The bottom should be golden brown and not stick to the pan. Wait another 30 seconds and check again before flipping.
Using a fish spatula, flip the fish over. Add more oil if the pan looks dry. Cook for another 3 to 5 minutes until it flakes.
Warm up the tortillas in a pan or microwave. Add fish to tortillas, breaking pieces apart if needed. Add toppings and serve.