In a small saucepan, gently warm the oysters in their juices for a minute or two before removing from the heat.
Put the shallot and champagne into a separate saucepan and bring to the boil, allowing to reduce in volume by half. Strain the oyster juice into the pan, continuing to boil until once again reduced by half.
Add the double cream and once simmering, whisk in 75g/3oz of the butter into the sauce a bit at a time. Finish with a squeeze of lemon juice, seasoning with salt and freshly ground black pepper. Keep the sauce to one side.
Divide the remaining butter between two frying pans and place both over a medium heat. Once sizzling, place the turbot in one pan and the shredded leek in the other. Season both with salt and freshly ground black pepper. Gently fry both for a few minutes, allowing the turbot to take on a light golden colour before turning. Cook until the leeks are softened.
Warm the sauce, adding the cooked oysters and chopped chervil. Divide the leeks between four bowls, placing a single oyster onto each. Place the turbot on top, presenting each with a remaining oyster before spooning the sauce over.