For the salmon, preheat the oven to 180C/350F/Gas 4.
Set the two fleshier pieces of salmon aside. Dice the tail piece of salmon.
Blend the diced salmon tail, shallots, cream, aniseed, egg and parsley in a food processor to a smooth paste. Season, to taste, with salt and freshly ground black pepper.
Turn out the blended salmon mixture into a large bowl. Add the small raw prawns and mix well until combined.
Rub the pastis all over the middle of each remaining whole piece of salmon. Make a 1cm/½in deep cut across the centre of each piece of salmon, then spread the salmon and prawn mixture all over the cut side of the fish.
Fold each piece of salmon in half so that the prawn mixture fills the salmon. Tie together with string to form two parcels.
Heat the butter in a flameproof roasting tin over a medium heat. Add the garlic and the salmon parcels, then fry until browned on all sides.
Transfer to the oven and roast for 7-8 minutes.
Add the Mediterranean prawns to the roasting tray, in their shells, and roast for a further 7-8 minutes, or until the salmon and prawns are cooked through.
Meanwhile, for the stir-fried sprouts, heat a wok over a medium to high heat. When the wok is hot, add the oil and heat until just smoking. Add the onions and Brussels sprouts and stir-fry for 2-3 minutes, or until just tender.
To serve, slice the filled salmon parcels. Divide the roasted salmon slices equally among three to four serving plates. Spoon the roasted prawns and stir-fried sprouts alongside. Sprinkle over the remaining parsley leaves.