Prepare chicken loaves:
1. In a saucepan cook onion in oil over moderate heat, stirring occasionally, until softened, and cool. In a large bowl combine well chicken, celery, parsle y, garlic, eggs, milk, cheese, bread crumbs, and onion mixture. Cover mixture and chill 1 hour.
2. Divide chicken mixture into 5 equal portions and form into small loaves.
3. In a large deep skillet cook garlic in oil over moderate heat, stirring, un til golden. Remove and discard garlic. Add chicken loaves and cook, turning o nce, until golden. Add mushrooms and Marsala and cook until liquid is reduced by half. Add chicken broth and cook, covered, 20 minutes, or until meat thermo meter inserted in center registers 80ºC . Transfer loaves to a platter and boil sauce until reduced by half. Whisk in butter and pour sauce over loaves. Ser ves 5 generously.