Make the salsa by combining the mango, red onion, coriander and lime zest and juice in a bowl. Season and set aside.
Tip the polenta onto a plate and season. Pour the eggs into a shallow bowl.
Season the fish, then dip each piece first into the egg, coating on both sides, then into the polenta, shaking off any excess.
Heat the oil in a large frying pan set over a medium heat. Carefully add the fish to the pan and cook for 3 minutes, turning halfway, until crisp and golden.
Meanwhile, heat the tacos according to pack instructions.
Make the guacamole. Roughly mash the avocados in a bowl. Add the onion, lemon or lime juice, olive oil and coriander, if using. Season and mix well.
To serve, add a spoonful of guacamole to each taco shell, followed by two pieces of fish. Top with the sliced radishes and shredded cabbage. Spoon over the salsa and offer sour cream on the side, if you like.