Preheat the oven to 220C/425F/Gas 7. Slice the tops off the rolls or brioches and scoop out most of the bread inside. Melt 55g/2oz of the butter. Brush the undersides of the lids and the hollows of the rolls inside and out generously with the melted butter.
Place the loaves on a baking sheet. Bake for about 10 minutes or until lightly brown. Meanwhile clean and open the oysters, drain off the liquor and reserve to one side.
While the loaves are browning, heat the remaining butter in a large frying pan. Sauté the oysters in the pan about 1 minute.
Remove the oysters from the pan with a draining spoon, cut them into 3 pieces and set aside. Add the reserved oyster liquor to the pan with the cream. Season with salt, pepper and nutmeg. Boil down steadily, stirring constantly, to form a very thick sauce, adding in the lemon juice.
Return the chopped oysters to the pan and gently reheat. Spoon the oysters and the sauce into the warmed rolls. Top with the lids and serve at once.