Tip the oil from the oyster tin into a saucepan, reserving the oysters. Add the butter and heat until melted and beginning to sputter. Add the onions and fry without coloring until soft, about 3 to 4 minutes.
Stir in the flour to form a paste, then gradually whisk in the cream a little at a time until smooth and creamy. Whisk in the water, nutmeg, and sea salt to taste. Remove from the heat.
Transfer the bechamel to a blender, add the reserved oysters and puree until completely smooth.
In a large bowl, mix the cooked kale thoroughly with the sauce and spoon into a baking dish.