Step 1Pat the chicken dry and put it on a plate. Combine the olive oil, onion, coriander, cumin, cayenne and 1/2 teaspoon salt in a small bowl. Rub the mixture evenly on all sides of the chicken, and inside a whole chicken. Let the chicken stand at room temperature to marinate briefly while you heat the grill; or cover the chicken and marinate it in the refrigerator for 2 to 4 hours.
Step 2If using a charcoal grill, heat the coals until they're covered with gray ash and have only a hint of red; then move them to the edge of the grill. Preheat a gas grill for indirect heat according to the manufacturer's instructions, then reduce the heat to medium (350 to 375 degrees). For a whole chicken, put a drip pan in the grill's center so it will catch the bird's juices. If the grill grate is adjustable, set it 6 inches above the coals.
Step 3Brush off the excess marinade and bits of onion clinging to the chicken. Discard any remaining marinade. Sprinkle the chicken with salt and pepper and set it on the grill. Cover and grill, turning the chicken over halfway through its estimated cooking time. Check for doneness: Whole chickens and dark meat pieces should measure 180 degrees at the thigh's thickest part and breast pieces should be 170 degrees. Or, with a thin skewer, pierce the thigh's thickest part near the bone; the juices running out should be clear, not reddish, and the meat should no longer be pink, but off-white. Breast meat should be off-white at its thickest part. To keep the chicken as succulent as possible, remove it immediately from the grill to a clean platter
Step 4Serve the chicken pieces with lemon wedges and mint sprigs. Let a whole chicken stand for 10 minutes before carving.