1. Preheat the oven to 450°F (230°C) and position a rack in the lower third of the oven.
2. Finely grate the zest of the lemon into a small bowl and reserve the lemon half.
3. Mix the softened butter with the minced herbs, minced white and light green scallion parts, lemon zest, salt, and pepper until well combined.
4. Divide the herb butter in half; wrap one half tightly and freeze for later use (optional).
5. Pour olive oil into a Dutch oven or roasting pan and swirl to coat the sides.
6. Spread some herb butter on one side of the bread and place it buttered side up in the pot.
7. Gently loosen the chicken skin over the breasts and drumsticks without tearing. Press most of the herb butter under the skin directly onto the meat. Pat the skin dry and smear remaining butter over it.
8. Season the chicken inside and out with salt and pepper. Stuff the cavity with the lemon half, herb stems, and reserved scallion greens.
9. Place the chicken breast-side up on top of the bread in the pot. Add the sliced onion and bay leaf to the pot. Pour the white wine or water around the chicken.
10. Roast uncovered for 50 to 60 minutes, checking and adding water if the pan juices run low. The chicken is done when internal temperature reaches 165°F (74°C) or juices run clear from a slit in the thickest part.
11. Remove the chicken to a cutting board and let rest about 10 minutes.
12. Skim fat from the pan juices, discard the bay leaf, and stir in sherry vinegar if desired. Serve chicken carved with the pan juices and jus-soaked bread.