Make the sauce first. Bring the oyster water to a light boil with the onions, salt, and Tabasco. Cook for about two minutes.
Meanwhile, melt the butter in another saucepan over low heat and stir in the flour. Make a blond roux. Strain the oyster water into the roux and whisk to blend. Add the Parmesan cheese and continue whisking until the cheese melts in. Remove from the heat.
Beat the egg yolks with the cream, and rapidly whisk this into the sauce. Keep the sauce warm.
Mix the salt and pepper into the flour, and lightly dust (don't dredge!) the oysters with the mixture. Pass the oysters through the beaten egg. Shake off the excess.
Mix the marjoram into the bread crumbs. Then dredge the oysters through the bread crumbs.
Heat about a half-inch of oil in a heavy skillet (cast iron is perfect) over medium-high fire, until a pinch of bread crumbs fries vigorously. Cook the oysters for about a minute and a half per side, or until the exterior is golden brown. Remove and drain on paper towels.
Stir the ham into the sauerkraut. Microwave on high for 99 seconds, or until steaming hot. Drain any water that may come out.
Place a heaping tablespoon of the sauerkraut on each oyster shell, or in little piles in an au gratin dish. Top the sauerkraut with a panneed oyster, and spoon some of the Mornay sauce over the oyster. Place the shells in a pan of rock salt, and run under the broiler long enough to just begin to brown the mornay--a minute or two. Serve with fresh grated horseradish.