Combine all the marinade ingredients and pour over the prawns. Cover and leave to marinate for a few minutes while you prepare the marrow.
Peel the marrow, cut in half lengthways and remove any seeds. Use a vegetable peeler to slice the marrow into thin slices of about 1mm.
Heat the oil in a heavy bottomed pan then stir in the garlic until the oil is fragrant. Add the marinated prawns and stir quickly until they turn pink.
Pour in the hot water and bring to the boil, skimming off any foam. Season with the Gia and the Fish Sauce then add the marrow slices and cook for about 5 minutes until soft.
Remove from the heat and sprinkle with the chopped spring onion to serve.