Heat butter in fry pan, add onions, and brown lightly.  Toss cheese with dry mustard.  Add cooked noodles to cooked onions and cheese; mix well. Place mixture in an ovenproof casserole.  Bake at 300 degrees F. for 20 to 30 minutes or until hot and bubbly.  Sprinkle top with  chopped chives before serving.