Heat a deep-fat fryer no more than a third full of oil to 180C. Or, heat the oil to a depth of 10cm in a pan until a cube of bread sizzles and turns golden in 30 seconds. Carefully lower 3-4 prawns into the oil at a time and deep-fry for 2 mins, or until crisp and golden. Transfer to a plate lined with kitchen paper using a slotted spoon, then season and sprinkle with the coriander leaves and lemon zest, if you like. Serve with the hot gumbo dip (see below) and some lemon wedges for squeezing ov