Melt butter in a large heavy saucepan over low heat; add flour and seasonings, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly.
Beat egg yolks until thick and lemon colored. Gradually stir one-fourth of hot white sauce into yolks; add to remaining hot white sauce.
Beat egg whites (at room temperature) until stiff but not dry. Gently fold egg whites and minced oysters into hot white sauce. Spoon mixture into a lightly greased 1 1/2-quart soufflé dish. Place dish in a 13- x 9- x 2-inch baking pan; pour hot water into pan to a depth of 1 inch. Bake at 350° for 45 minutes or until puffed. Serve immediately.