First make the bisque, by putting the diced carrots, celeriac, shallots, celery, leeks, garlic and tomatoes and roast off until they are golden brown. Add the scallop corals and tarragon and cook off for another 5 minutes. Deglaze with the port and brandy and flambé to burn off the alcohol. Then add the water and bring to the boil, then turn off the heat and leave the bisque to cool for 20 minutes. Finally pour in the cream and strain through a sieve. Transfer the soup into a pot and reheat when ready to serve.
To make the caramelized onions, and place in a hot pan and cook very slowly for at least one hour or until the onions are soft and translucent - you can add a teaspoon or 2 of water to stop the onions browning. Season to taste.
Season the scallops and cook in hot pan for about 1 minute on each side. Drizzle with some lemon juice.
To serve, place some slow cooked onions in the centre of the plate. Pour some hot bisque around the onions and place the scallop on top.