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1
Heat the oven to 500 degrees F and arrange a rack in the lower third.
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2
Mound the onions in the center of a roasting pan.
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3
Slice one of the lemons into thin rounds and place on top of the onions.
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4
Slice the remaining lemon in half; set aside.
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5
Rub the chicken all over with salt and pepper.
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6
Season inside the cavity with salt and pepper, then place the reserved lemon halves and herbs, if using, inside.
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7
Place the chicken, breast side up, on top of the onions and lemon slices and tuck the wings back and underneath.
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8
Roast 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).
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9
Insert 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.
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10
Transfer the chicken to a cutting board or serving platter and let it rest for at least 10 minutes before carving.
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11
Meanwhile, transfer the sliced lemon rounds from the roasting pan to a serving plate; set aside.
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12
Place 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.
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13
Simmer until reduced by half.
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14
Remove from heat.
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15
Using a slotted spoon, transfer the onions to the plate with the lemon slices.
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16
Taste and season the sauce with salt and pepper as needed.
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17
Serve the chicken with the lemon rounds, onions, and sauce.
