Saute scallions and 1 cup of mushrooms in butter. Remove from heat and mix in sour cream. Combine remaining ingredients except parsley in a bowl and mix. Put 1/2 mixture in loaf pan, make an indentation lengthwise along center of the beef, spoon mushroom mixture into indentation. Top with remainging beef mixture. Shape into loaf, completely enclosing the filling. Bake at 350* for 1 hour. Garnish with parsley.