1.Place one chicken, breast-side down, on a chopping board. Cut down both sides of the backbone using kitchen scissors; discard backbone (or reserve for another use, see tips). Turn chicken over; press down firmly and steadily onto the breast bone to break so the chicken lies flat. Pat chicken dry with paper towel. Repeat with remaining chicken.
2.Heat 1 tablespoon of the oil in a 7 litre (28-cup) cast iron or other flameproof casserole dish (see tips) over high heat. Add one chicken, breast-side down; cook, pressing legs down so they touch the base of the dish, for 10 minutes or until chicken is golden brown. Transfer to a large plate or tray. Repeat with another tablespoon of oil and remaining chicken; transfer to a large plate or tray.
3.Add mushrooms and shallots to same dish; cook, stirring, for 5 minutes or until golden brown. Add verjuice, scraping base of dish; cook for 1 minute or until simmering. Return chickens to dish, side-by-side; top with leek, chopped tarragon and stock; season. Reduce heat to low; cook, covered with a tight-fitting lid, for 1 hour or until chickens are cooked through.
4.Heat remaining oil in a small frying pan over low heat; cook extra tarragon for 20 seconds or until crisp. Drain on paper towel.
5.Stand casserole dish, covered, for 5 minutes. Transfer one of the chickens to a serving platter; cut into four pieces. Spoon over half the mushrooms, leek and sauce and top with crispy tarragon.
6.Cut remaining chicken into four pieces; transfer to an airtight container with remaining vegetables and sauce. Cool, then store (see > store it).
We used a 25cm x 33cm (10in x 13¼in) oval cast iron casserole dish. Freeze the unused backbones to make your own chicken stock (page 230).
> serve it Serve with roast potatoes (page 23) or, if you prefer, lyonnaise potates (page 22).
> store it Refrigerate chicken stew in an airtight container for up to 2 days. Freeze for up to 3 months; thaw in the fridge, then reheat in