CHICKEN POT-AU-FEU

Steps:

• Poach chicken:
• Pat chicken dry and sprinkle inside and out with salt and pepper. Stuff cavity with thyme and chill chicken, loosely covered, at least 3 hours (to allow seasoning to permeate meat).
• Transfer chicken to a 7- to 8-quart heavy pot, then add stock, water, onion, garlic, celery rib, and bay leaf and bring to a boil. Reduce heat and gently simmer chicken, covered, until juices run clear when a thigh is pierced with a skewer and thermometer inserted into thickest part of a thigh (do not touch bone) registers 170°F, about 45 minutes. Transfer chicken to a large platter and let stand, loosely covered with foil, 20 minutes. Reserve cooking liquid in pot, uncovered, and discard onion, celery, garlic, and bay leaf.
• Prepare leeks and potatoes while chicken cooks:
• Starting about 1/2 inch from root end, make a lengthwise cut through middle of each leek (keep leek intact). Wash leeks well under cold running water and drain. Halve potatoes.
• Cook vegetables while chicken stands:
• Bring chicken cooking liquid to a boil, then add leeks, potatoes, carrots, parsnips, and celery root and simmer, partially covered, until tender, about 20 minutes. Carefully transfer vegetables with a slotted spoon to platter with chicken and keep warm, loosely covered with foil. (Reserve cooking liquid for another use if desired cool, uncovered, then chill, covered.)
• Make sauce while vegetables cook:
• Mash garlic and capers to a paste with salt using a mortar and pestle (or mince and mash with a large heavy knife). Transfer to a food processor and pulse with parsley and watercress until herbs are finely chopped. With motor running, add oil in a slow stream and process until smooth. Season sauce with salt and pepper.
• Serve chicken and vegetables with sauce.