Thinly slice 1 onion. Strip the leaves from 2 sprigs of rosemary and finely chop the leaves, reserving the sprigs. Grate the zest from 1 lemon.
In a small bowl, combine the sliced onion, chopped rosemary leaves, lemon zest, vinegar, mustard, the juice of the zested lemon, and the salt and pepper.
Cut the remaining onion and lemon in half. Pierce each first with a small, sharp knife so that the skewer is easily inserted. Insert the sprigs of rosemary through the onion and lemon halves.
Rub the mustard mixture over the chicken and inside the cavities. Insert a rosemary "skewer" into each cavity. Place the chicken in a shallow pan, cover, and refrigerate for at least 2 hours or overnight.
Prepare a charcoal or gas grill, arranging the coals for indirect cooking. Lightly spray the grill rack with vegetable oil spray. The coals should be moderately hot.
Place the chicken over the cooler part of the grill, cover and roast for 1 1/2 hours to 1 hour 45 minutes, turning at least 3 or 4 times during grilling, until the juices run clear when the thigh is pierced with a fork or sharp knife. Add fresh coals to the fire as necessary to maintain a moderate heat. If using a gas grill, adjust the burner furthest from the chicken to medium. Let the roast chicken sit for about 10 minutes. Remove the onion and lemon from the cavities before serving.
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