1.Pound coriander roots, garlic and a pinch of sea salt to a fine paste with a mortar and pestle. Add fish sauce, lime juice and sweet chilli sauce, stir to combine and set aside.
2.Heat oil in a large frying pan over high heat and cook prawns in batches, stirring, until just cooked and golden-brown (2-3 minutes). Transfer to a plate.
3.Toss watermelon, grapefruit, cashew nuts, herbs, shallots and lime leaves in a bowl, add prawns, toss to just combine and serve scattered with fried shallots.