Combine ingredients in a small bowl, season to taste and refrigerate until required.
Combine polenta and flour in a bowl, season to taste.
Place buns on a tray and transfer to oven to warm through (5 minutes).
Heat oil in a deep-sided saucepan over medium-heat or in a deep-fryer to 180ºC.
Drain prawns from buttermilk mixture (discard), shaking off excess, dip in polenta mix, shake off excess then fry in batches until golden and crisp and cooked through (1½ -2 minutes).
Transfer to a plate lined with kitchen paper. Repeat with remaining prawns and season to taste.
Spread rolls with remoulade, top with iceberg, white onion and prawns. Serve with fresh coriander and lemon if desired.