Peel and devein the prawns, leaving the tails intact. Put the chilli powder, turmeric, cumin, mustard, garlic, ginger and chopped red onion in a blender, food processor or pestle and mortar and process to form a paste. If necessary, add a little water.
Heat the oil in a karhai or heavy-based frying pan and fry the sliced onion. When it starts to brown, add the curry paste and fry until