Heat coconut oil in a medium saucepan on a high heat. Add the onions, ginger, chilli flakes, lemongrass, Simon Gault Fish Stock concentrate and tom yum paste and cook until most of the liquid has evaporated and the mixture begins to caramelise. This should take about 6-8 minutes. Stir in the tomato paste, tomato purée and Gault's ketchup. Add the coconut sugar, soy sauce, white vinegar, salt and bring to a gentle simmer for 8-10 minutes. Stir in the cornflour and water mixture until the sauce thickens. Set aside for the prawns. Heat the coconut oil in a large frying pan over a high heat (or use a BBQ if you prefer) until the oil begins to shimmer. Add the prawn cutlets and cook for 2 minutes until they just start to colour then add 2 cups of Lemongrass & Chilli Tomato Sauce. Mix prawns through the sauce and cook for a further 2-3 minutes or until the prawns are cooked through and the sauce is hot. On a serving platter place iceberg lettuce leaves and top with prawns and lemongrass and chilli tomato sauce. Garnish with chopped parsley.