1. Position a rack in the lower third of the oven and preheat to 450°F (232°C).
2. Grate lemon zest into a small bowl and reserve the halved lemon for later use.
3. Combine the softened butter with minced herbs and scallions (white and light green parts), season with salt and pepper, and mash until thoroughly blended.
4. Divide the herb butter in half; wrap one portion tightly in plastic wrap and freeze for future use.
5. Pour olive oil into a Dutch oven or oven-safe pot and coat the sides.
6. Spread some herb butter on one side of the bread and place it buttered side up in the pot to serve as a flavorful roasting rack.
7. Gently loosen the chicken skin and spread the majority of the remaining herb butter under the skin against the meat, then pat the skin dry and spread the rest of the butter on top.
8. Season the chicken inside and out with salt and pepper. Stuff the cavity with herb stems, reserved scallion greens, and place the squeezed half lemon inside.
9. Place the chicken breast side up on the buttered bread. Add sliced onion and bay leaf to the pot.
10. Pour the white wine or water around the chicken to keep moisture during roasting.
11. Roast uncovered for 50 to 60 minutes, adding water as needed if pan juices run low. The chicken is done when a meat thermometer inserted into the thickest part of the thigh reads 165°F (74°C) or when juices run clear upon cutting.
12. Remove the chicken and let it rest on a cutting board for about 10 minutes.
13. Skim fat from pan juices, discard bay leaf, and stir in sherry vinegar if using.
14. Carve the chicken and serve with the pan juices and bread soaked in the flavorful jus.