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Mix the sauce ingredients together in a small bowl. Set aside.
Heat the oil in a large wok. Sauté the lemongrass and ginger until golden. Add the pineapple, beans and cherry tomatoes, and stir-fry for 3-5 mins until the beans are just cooked. Add the prawns and the sauce. Stir-fry for another 3-5 mins until the prawns are cooked, then throw in most of the basil leaves. Serve with lime wedges and the remaining basil leaves scattered over.