Warm the oysters in their juices in a small saucepan for two minutes.
Boil the shallot and champagne in a saucepan until reduced by half.
Strain the oyster juice into the champagne and reduce by half again.
Simmer the cream and whisk in 75g of butter gradually.
Finish the sauce with lemon juice and seasoning, then set aside.
Melt remaining butter in two frying pans over medium heat.
Fry the seasoned turbot and shredded leeks in separate pans until cooked.
Reheat the sauce, adding the oysters and chopped chervil.
Arrange the leeks and oysters in bowls, top with turbot, and spoon sauce over.