Peel and clean prawns, leaving tails intact and reserving the heads and then using a sharp knife, butterfly the prawns by slicing along the underside, from head end to tail, without cutting all the way through.
Heat the oil and butter in a large saucepan over medium high heat and add the garlic and reserved prawn heads, and cook, stirring for 1 minute and then add stock and then the water and bring to the boil.
Once the stock comes to the boil, remove pan from the heat and strain liquid through a colander into a bowl and discard the prawn heads and return the stock to the pan.
Add pasta sauce and mussels and stir to combine and cover and bring to the boil over hight heat and cook for 1 minute or until shells open.
Add the prawns and fish and cook a further 2 minutes or until the seafood is cooked through and then stir in the parsley and season with salt and pepper.
Serve with lemon wedges and crusty bread if you wish.