1. In a heavy flameproof 4- to 5-quart casserole, heat the olive oil, tipping the casserole to coat the bottom evenly and turning the heat down to moderate if the oil begins to smoke.
2. Pat the chicken dry with paper towels and brown it in the oil, a few pieces at a time. Start the pieces skin side down and turn them with tongs. Add the wine, vinegar, water, onions, carrots, leek, salt and bouquet, and bring to a boil over high heat.
3. Reduce the heat to low, cover the casserole, and simmer undisturbed for 30 minutes, or until the chicken is tender but not falling apart. Remove the bouquet, and arrange the chicken pieces in a deep serving dish just large enough to hold them snugly in one layer.
4. Pour the cooking liquid with the vegetables over the chicken. Decorate the top with the lemon slices, and cool to room temperature. Cover the dish and' refrigerate for at least 6 hours, or until the cooking liquids have jelled.
5. Serve on chilled plates, as the first course or appetizers.
