To make larb, place onion, garlic, lemongrass, lime leaves and ginger in a food processor and process until it forms a paste. Heat oil in a large, non-stick frypan or wok. Cook paste for 1 minute until fragrant. Add mince, breaking up with your spatula into small pieces as it cooks and browns for 5–7 minutes. Add prawn and chilli, stirring for 3–4 minutes until prawn is cooked. Remove from heat and add lime juice, fish sauce, sesame oil and mint and stir through. Arrange banana leaves in the centre of a large serving board. Place small piles of bean sprouts, cashews, chilli and cos leaves around edge of board. Spoon larb into the centre of banana leaves and sprinkle with green onion. Serve immediately with lime halves.