Preheat the oven to 400°. Bring a large pot of lightly salted water to a boil. Submerge the spinach in the water and drain immediately. Melt the butter in a large skillet and sauté the scallions, shallots, and garlic over moderate heat until wilted. Press any excess liquid from the spinach and add it to the pan. Fold in the parsley and the remaining filling ingredients. Puree the mixture in a food processor. To assemble: cover 1 or 2 ovenproof pans with ½ inch of rock salt. Nestle the oysters in the salt; it will keep them level. Spread 1 tablespoon of the spinach mixture over each oyster. Bake for 10 minutes, or until the edges of the oysters begin to curl. Serve immediately.