First make the chicken stock. Place the chickens, chopped vegetables, garlic and herbs in a large pan. Cover with cold water and bring to the boil. Skim off any fat that rises to the surface, then leave to simmer for 1½-2 hours.
Strain the stock through a colander set in a bowl, then pass through a sieve. You need 700ml/1¼ pints of stock for the soup; keep the remainder for another use.
Remove the 'oysters' (the small oyster-shaped piece of meat above the thigh) from the chicken back portions and leave to one side for use later as a garnish. Discard all the remaining contents of the colander.
For the soup, melt the butter with the olive oil in a saucepan. Add the onion and slowly cook without colouring. After a couple of minutes add the leeks, then cover and sweat for about five minutes over a low heat. Do not allow the vegetables to colour.
Add the potatoes and bouquet garni and cook uncovered for two minutes, then pour in the hot stock. Bring to the boil and cook until the potatoes are soft.
Remove the bouquet garni, then carefully blitz the soup in a food processor or blender. Pour into a bowl and set over ice to cool. Chill in the fridge until ready to serve.
Remove the rock oysters from their shells and lightly poach in the fish stock. Drain and allow to cool.
To serve, place the sea oysters and chicken 'oysters' in large soup bowls and ladle the soup on top. Finish with a sprinkling of chives.
