Poulet En Casserole

• 12 onions (whole white)
• 12 potatoes (white)
• 4 carrots
• 2 garlic (cloves)
• 1 broiler-fryer
• 2,800 gr casserole
• 1,700 gr flame proof casserole
• 240 ml dry wine (white)
• 10 ml vegetable oil
• 9 gr butter