Cut the salmon into three pieces. Put the two fleshier pieces aside and dice the tail end into small cubes.
Preheat oven to 180C. Process the diced salmon, shallot, half the parsley, cream, aniseed and egg in a food processor until smooth, then season to taste.
Combine the salmon mixture and the small prawns in a bowl.
Rub the middle of the remaining two pieces of salmon with the pastis. Make 1cm cuts in the salmon and spread with the salmon-prawn filling. Close up like a book and tie together with string.
Heat the butter in a large flameproof roasting pan. Add the garlic and then the salmon parcel and brown on all sides over medium heat. Bake in the oven for 7 minutes, then remove and add the prawns and bake for a further 7 minutes or until just cooked. Scatter over the rest of the parsley before serving.