Add the 3 cups cold water to the oysters; refrigerate at least one hour. Strain and reserve oyster water and oysters in refrigerator until ready to use.
In a large soup pot combine the 8 quarts water, olive oil, 3 tablespoons of the Vegetable Magic® and lemons. Cover and bring to a boil over high heat.
Meanwhile, cut the stems off the artichokes (use a stainless steel knife so the knife and artichokes won't discolor); reserve the stems. Trim artichoke tops down about ½ inch. Add artichokes (top down) and stems to the boiling water; cover pan and boil just until leaves can be pulled off easily, about 25 minutes, stirring occasionally and rotating artichokes once or twice during cooking; drain. Cool slightly, then gently pry open center leaves of artichokes just enough to remove the small purple-tipped leaves and the small adjacent leaves covering the fuzzy choke. With a teaspoon carefully scoop out choke from the center and discard, leaving the artichoke heart intact at the bottom. Chop edible parts from the reserved innermost leaves. Cut off and discard the stringy skin and the ends of the stems; then chop the tender center of the stems. Chopped pulp should come to about ⅔ cup. Clip the pointed tips from the outer leaves of the artichokes with scissors and discard. Set artichokes and chopped pulp aside.
In a large heavy skillet (preferably nonstick) melt the butter over high heat. Whisk in the flour with a metal whisk until smooth. Add the green onions, reserved artichoke pulp, minced garlic, the remaining 1 teaspoon salt, the ground peppers and thyme; saute for about 2 minutes, stirring frequently. Add the reserved 3 cups oyster water and bring to a boil while whisking frequently with a metal whisk, about 5 minutes; continue cooking 5 minutes more, stirring occasionally. Add the oysters and cook about 3 minutes. Stir in the cream and continue cooking until sauce is creamy and thick, about 8 to 10 minutes, stirring occasionally. Remove from heat.
Place the artichokes upright in an ungreased baking pan just large enough so the 6 artichokes fit in snugly. Open each artichoke slightly if necessary. Spoon about ½ cup sauce into the center of each artichoke, giving each an equal share of oysters; sprinkle 1 teaspoon Parmesan over the sauce. Bake at 400°F until cheese starts to brown, about 35 minutes. Remove from oven and serve immediately. To eat, pull outer leaves off artichoke and dip into the sauce in the center.