1Heat the oven to 500°F and arrange a rack in the lower third.
2Mound the onions in the center of a roasting pan. Slice one of the lemons into thin rounds and place on top of the onions. Slice the remaining lemon in half; set aside.
3Rub the chicken all over with salt and pepper. Season inside the cavity with salt and pepper, then place the reserved lemon halves and herbs, if using, inside. Place the chicken, breast side up, on top of the onions and lemon slices and tuck the wings back and underneath.
4Roast until the chicken is golden brown all over and the juices run clear from the legs when cut with a knife, about 45 minutes (or 10 minutes per pound).
5Insert the handle of a wooden spoon inside the cavity of the chicken, then lift and tilt it slightly to let the juices run back into the roasting pan. Transfer the chicken to a cutting board or serving platter and let it rest for at least 10 minutes before carving. Meanwhile, transfer the sliced lemon rounds from the roasting pan to a serving plate; set aside.
6Place the roasting pan across two burners over medium heat, add the vermouth, and scrape up any browned bits from the bottom of the pan with a wooden spoon. Simmer until reduced by half. Remove from heat. Using a slotted spoon, transfer the onions to the plate with the lemon slices. Taste and season the sauce with salt and pepper as needed. Serve the chicken with the lemon rounds, onions, and sauce.