In a bowl, combine melted butter, bread crumbs, 1/2 cup of the Parmesan cheese, herbes de Provence, and a pinch of salt and pepper. Set aside.
Place the onion, bay leaf, clove, milk and Tabasco in a small heavy saucepan over medium heat. When the mixture comes to a boil, remove the pan from the heat and strain. Reserve and keep warm.
Melt the butter in a medium saucepan over low heat. When the butter is foamy, add the flour and make a blond roux. Remove from the heat when the first signs of browning appear. Whisk in the milk until the the sauce is fluffy and dry. Add the remaining Parmesan cheese, horseradish and nutmeg. Season with salt, pepper, and Tabasco sauce.
Fill a medium saucepan with water and bring to a boil. Fill a bowl with cold water and ice cubes. Poach the oysters in the boiling water until the edges curl. Remove with a slotted spoon and plunge immediately into the ice water to stop the cooking. Drain and set aside.
Preheat the oven to 375 degrees. Place a layer of rock salt about 1/2 inch deep in metal baking pans. Press the shells onto the salt. Spoon a tablespoon of sauce into each shell, then top with an oyster. Cover with another tablespoon of sauce, then sprinkle the bread crumb mixture from the first step on top. Bake at 375 degrees for ten minutes, or until a golden crust forms.