On a sheet pan, combine the fish with 1 teaspoon Kosher salt (double this if doubling the recipe), a generous grinding of freshly ground black pepper, and a third of the white rice flour. Toss to coat evenly.
In a medium bowl, make the batter. With a fork, mix together the remaining white rice flour, the corn starch, baking soda, egg, garlic powder, chili powder, ice water, and 1 teaspoon Kosher salt (double this amount of Kosher salt if doubling the recipe) until just combined. No need to overmix. Batter should be thick enough to coat pieces of fish, but not too thick that you can’t easily dip a piece of fish in.
Fill up a large, high-sided pan halfway with neutral frying oil. Heat over medium-high heat until hot enough to start frying (you can tell if the oil is hot enough if a little bit of flour sprinkled in sizzles). Reduce heat to medium. Prepare a sheet pan with a wire cooling rack on top.
Add a third of the fish to the batter and toss to coat. Using a fork, gently lift each piece of fish from the batter, letting any excess drip off, and place into the hot frying oil. Fry for 2-3 minutes until lightly golden, flipping halfway. Using tongs, transfer fish to wire rack-lined sheet pan and repeat with remaining fish pieces, in batches. Let fish cool at room temperature for at least 10 minutes or up to 2 hours. Turn off heat but keep frying oil in pan for the second fry.
When ready to eat, heat tortillas using your oven or open flame. If using the oven, wrap all tortillas in tinfoil and place in 350°F oven until heated through, 10-15 minutes. If using an open flame, such as a grill or gas stovetop, turn on the heat to medium-high. Using tongs, lightly char each of the tortillas on both sides until warmed. Place into tinfoil pouch to keep warm. (Using your oven is easier, but I love the charred flavor from the open flame.)
Meanwhile, make the slaw. In a medium bowl, combine the coleslaw mix, mayonnaise, lime juice, and a pinch of Kosher salt and freshly ground black pepper (TIP: I find it easiest to mix using my hands). Taste and season with more salt and pepper, if needed.
Heat oil again over medium-high heat. Once hot, fry fish (in batches if needed) for 1-2 minutes, until more golden and crispy. Transfer back to rack.
ALTERNATIVELY - You can reheat in an air fryer. Heat at 400°F and cook for 2-4 minutes, until batter is crisped up again.
Make the tacos. Place as much slaw as desired into each taco, followed by a piece of the fried fish. Drizzle with Aji Green Sauce, then sprinkle with radish slices and cilantro leaves and serve.