Combine polenta and chilli powder in a bowl and season with salt. Cut fish into 2cm-thick strips and toss in polenta to coat. Whisk sour cream, lime juice and 2 tbs water together in a small bowl and set aside.
Heat oil in a large non-stick frying pan over medium heat. Cook fish, in batches, for 2 minutes on each side or until crisp and just cooked through. Drain on paper towel.
Meanwhile, lightly toast tortillas on a chargrill or in a dry frying pan.
Fill tortillas with cabbage, avocado and fish. Top with sour cream mixture and coriander. Fold in half and serve with lime wedges.