Using ingredient note if using mushrooms in place on onions

French-style Baked Chicken

• 12 onions (white peeled)
• 12 potatoes pared
• 12 mushrooms (whole)
• 4 sprigs parsley
• 2 celery tops with leaves
• 1 onion (yellow)
• 1 parsley for (chopped garnish)
• 1 note if using mushrooms in place on onions
• 1,400 ml broiler-fryer chicken
• 28 gr butter