This classic French recipe calls for poussins, which are very small young chickens and produce tender meat.
Rinse poussins; pat dry. Season with salt and pepper. Place 4 thyme sprigs into each cavity. Carefully tie legs together with kitchen twine. Set aside.
Bring a medium saucepan of water to a boil. Add onions, and cook until skins soften, about 3 minutes. Remove onions with a slotted spoon, and rinse. Trim root ends, and remove skins.
Stir together stock, wine, onions, carrots, potatoes, celery, garlic, peppercorns, and 2 teaspoons salt in a large, heavy stockpot. Bring to a boil over medium-high heat. Reduce heat to medium-low. Gently add poussins, breast side down. Cover, and cook 1 hour. Place a poussin in each of 4 serving bowls; divide broth and vegetables evenly among the bowls. Garnish each bowl with chervil, and sprinkle with pepper.