Fill a large sauce pan with water to three quarters full, put it on the heat and bring it to the boil. Add the salt. Place a large bowl in the sink and fill it with ice and cold water. Carefully drop a handful of the prawns into the boiling water for thirty seconds then lift them out with a slotted spoon and drop them into the iced water to arrest the cooking. Remove from the bowl and put them onto a clean tea towel to drain. Continue until you have blanched off all of the prawns. Peel the prawns keeping the prawn tails and the shells.
Using a piece of butcher’s string, tie the herbs and star anise between the two pieces of celery to make a bouquet garni.
Heat the butter in a large heavy-bottomed pan over a medium heat and then add the onion, carrot, and leek and cook them until they are beginning to caramelise and turn golden brown, between twenty and thirty minutes. Then add tomato purée and flour and cook for two minutes, stirring continuously.
Add the prawn shells, Pernod, pepper and fish stock stir well and bring to the boil. Then turn down the heat add the bouquet garni and simmer it for one hour. When cooked remove from the heat and allow to cool a little. Remove the bouquet garni and ladle some of the contents of the pan into a liquidiser, until the liquidiser is no more than one quarter full. Blend it thoroughly for three minutes.
IT IS IMPORTANT TO FOLLOW THE MANUFACTURERS INSTRUCTIONS FOR THE LIQUIDISER ESPECIALLY WHEN YOU ARE PROCESSING HOT LIQUIDS. IT IS BEST TO DO SEVERAL SMALL BATCHES THEN OVERFILL YOUR LIQUIDISER.
Pour the contents of the liquidizer trough a chinois, a very fine sieve into another large, heavy-bottomed pan. Use a ladle to push the bisque through leaving the ground shells. Continue until all of the bisque has been blended and pushed through the sieve.
Return the soup to the heat and bring back up to a gentle simmer. Roughly chop the prawn tail meat and add to the soup and cook for a couple minutes. Add the cream and season it with salt & pepper and serve when hot.