Cook onion in bacon drippings until tender; stir in flour and cook for one minute, stirring constantly to make a roux. Gradually add chicken broth, celery; leeks and butter. Cook, stirring constantly until thickened and bubbly (use a wire whisk). Put in the baked potatoes, bacon, parsley and half & half; cook for 10 minutes. Stir in cheese and sour cream.