From 1890:

Blanch a dozen oysters in their own liquor, salt and remove the oysters, add a tablespoonful of butter, the juice of half a lemon, a gill of cream, and a tablespoonful of flour. Beat up the yolk of one egg while the sauce is simmering; add the egg and simmer the whole until it thickens. Place the oysters in a hot dish, pour the sauce over them, sprinkle a little chopped parsley on top, and send to table. To blanch an oyster is to cook it till it puffs up.