Combine capsicums, onion and pineapple, lime juice and cayenne, if using, in a large bowl. Set aside.
Combine polenta, cumin and paprika in a shallow bowl. Season with pepper. Place egg in another shallow bowl. Working in batches, dip fish in egg, then transfer to polenta mixture to coat.
Heat half the oil in a large frying pan over medium heat. Cook half the fish for 5 minutes, turning occasionally, until golden and cooked through. Drain on a paper towel-lined plate. Repeat with remaining oil and fish.
Warm tortillas according to packet instructions. Divide fish among tortillas, dollop over yoghurt, scatter over spinach and spoon over salsa. Serve tacos with extra lime.