Preheat oven to 450 degrees.
In a small skillet, melt 2 tablespoons of the butter over medium heat and saute' the mushroom until tender. Drain and set aside.
Drain the oysters, reserving 1 cup of the liquor (or substitute milk or chicken broth). Cover a large baking dish (or as many baking dishes as you need) with rock salt. Fit as many oyster shells as you can in each dish. Place an oyster in each shell. (You may also use ramekins, placing 4 oysters in each one)
Melt the remaining 6 tablespoons of butter in a 9 inch skillet over medium heat. Saute' the green onions and garlic for 5 minutes. SLowly add the flour and stir to make a smooth mixture. Remove from heat.
In a small bowl, beat the egg yolks, and stir in the parsley and cream. Stir the yolk mixture into the green onion and flour mixture, blending them well. Add the 1 cup of oyster liquor and cook over low heat, stirring, until it becomes a thick, creamy sauce.
To the sauce, add the drained chopped mushroom, cheese, and salt. Stir until the cheese is melted and well blended.
Spoon this sauce over the oysters and sprinkle with bread crumbs. Bake for 15 minutes, or until browned. Serve immediately with lemon wedges.