WASH AND BAKE POTATOES. COOL AND CUT IN HALF LENGTHWISE. SCOOP OUT PULP AND SET ASIDE. MELT BUTTER IN HEAVY SAUCEPAN OVER LOW HEAT. ADD FLOUR, STIRRING TIL SMOOTH. COOK 1 MINUTE, STIRRING CONSTANTLY. GRADUALLY ADD 6 CUPS MILK. COOK OVER MEDIUM HEAT, STIRRING CONSTANTLY, UNTIL MIXTURE IS THICKENED AND BUBBLY. ADD POTATO PULP, SALT, PEPPER, AND 1 1/2 CUPS CHEESE. COOK UNTIL THOROUGHLY HEATED. ADD SOUR CREAM. ADD EXTRA MILK TO OBTAIN DESIRED CONSISTENCY. GARNISH WITH GREEN ONION, BACON AND CHEESE.