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Tip the shallots into a small bowl with the vinegar and a pinch each of sugar and salt. Scrunch together with your hands, set aside and leave to pickle while you continue.
Mix the mayonnaise, sriracha, fish sauce and lemongrass paste together. Will keep covered and chilled for up to three days. Add the cucumber and prawns, and mix to combine. Separate the lettuces into individual leaves and spoon a tablespoon of the prawn cocktail into each lettuce cup. Top with the pickled shallots and crispy onions. Serve immediately.