Place unopened can of madrilène in refrigerator 2 hours or until partially set. (Madrilène should resemble consistency of unbeaten egg white.) Pour madrilène into a medium mixing bowl; add sour cream, beating with a wire whisk until well blended. Cover and refrigerate 4 to 5 hours or until mixture thickens and begins to jell.
Spoon into chilled serving bowls. Garnish with caviar and parsley or watercress. Serve as an appetizer.