Puree the oysters in a blender with a bit of the nectar, then pour that mess into a saucepan along with the pureed spinach. Add the milk, half-in-half, seasonings and butter and heat but do not boil. Mix cornstarch with a little cold water, stir into soup and let everything simmer about 15 minutes, making sure it does not boil. Whip cream, ladle soup into four ovenproof bowls, top each with a generous portion of cream and put under an oven broiler just until you have a golden pool of cream floating atop each bowl.