To make the chilli and garlic marinade, combine the ingredients in a jug and whisk well to combine. Season with a generous pinch of salt and black pepper and set aside until needed.
To prepare the prawns, uncurl a prawn, so it sits on your working surface belly-side up. Use a sharp knife to make an incision from the head to the tip of the tail, cutting through the meat but not all the way through the shell. Open the prawn up and flatten it out. Devein the prawns and arrange flesh-side up on a large baking tray lined with baking paper. Spoon the chilli and garlic marinade over the prawns.
Heat a chargrill pan with a lid or a hooded barbecue over medium heat, then grill the prawns, shell side-down. Cover and cook, without turning for 3-4 minutes, or until just cooked through.
Transfer the prawns to a large serving plate. Sprinkle with extra parsley, chilli and serve with lime cheeks and crusty bread on the side.