1. Preheat the oven to 450°F (230°C) and position the rack in the lower third.
2. Grate lemon zest into a bowl and set aside the lemon halves for juice and cavity use.
3. In the bowl, combine softened butter, minced fresh herbs, minced scallions (white and light green parts), salt, pepper, and lemon zest until well blended.
4. Divide the herb butter into two halves; wrap one half in plastic wrap and freeze for later use.
5. Pour olive oil into a Dutch oven or roasting pot and coat the sides.
6. Spread some herb butter on one side of the stale bread and place it buttered side up in the pot.
7. Gently loosen the chicken skin from the meat on breasts and drumsticks, then push most of the herb butter under the skin, spreading evenly.
8. Pat the chicken skin dry and smear the remaining herb butter on the outside of the skin.
9. Season the chicken inside and out with salt and pepper. Stuff the cavity with the reserved herb stems and scallion greens; place half the lemon inside as well.
10. Squeeze lemon juice over the chicken, then set the chicken breast side up on the bread in the pot.
11. Arrange sliced onion and bay leaf around the chicken and pour white wine or water around it.
12. Roast uncovered for 50 to 60 minutes, adding water if pan juices run low. The chicken is done when internal temperature reaches 165°F (74°C) or juices run clear.
13. Remove chicken and rest for 10 minutes on a cutting board.
14. Skim fat from pan juices, discard bay leaf, and stir in sherry vinegar if desired to make a pan sauce.
15. Carve the chicken and serve with pan juices and the bread soaked in the flavorful jus.