Put the flour in a strong freezer bag and season with salt and black pepper. Pour the beaten egg into a shallow dish. Put the breadcrumbs (or breadcrumb and polenta mixture) in a separate shallow dish.
Gently toss each fish piece in the seasoned flour bag until evenly coated, then dip into the beaten egg then the breadcrumbs until evenly coated. Put the prepared fish pieces on a plate.
Heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. Add the fish fingers carefully and fry for 5–6 minutes, turning occasionally until golden brown and crisp on all sides. (You may need to do this in batches.) Remove the fish with a spatula and drain on kitchen paper.
Bring a pot of water to the boil, and cook the frozen peas for 2-3 minutes or until soft. Drain.
Serve the fish fingers with the peas.