The weight of a brick on the flattened chicken pushes out the fat and allows the skin to cook to a delightfully crisp texture. If you do not have a brick, weighting the chicken with a large, heavy cast iron skillet will work beautifully, too.
Using heavy-duty kitchen shears or a sharp knife, cut and remove the backbone from the chicken. Discard the backbone. Pull the wings back and fold under the first joint. Press chicken evenly so it will lay as flat as possible, skin side up. Place chicken in a 3-quart rectangular baking dish. In a small bowl combine oil, lemon juice, sage, pepper, and salt. Brush mixture evenly over all of the chicken. Loosely cover and chill in the refrigerator for 2 to 3 hours.
Wrap a large heavy brick with foil. Remove the chicken from the refrigerator; let stand at room temperature for 20 minutes while preheating the grill.
For a charcoal grill, arrange medium coals around a drip pan. Test for medium-low heat over the coals. Place chicken, skin side down, directly over medium coals. Place brick on top of chicken. Cover and grill for about 8 minutes or until chicken is nicely browned. Remove brick and turn chicken; move chicken to indirect heat and place brick on top again. Cover and grill for 30 to 40 minutes more or until chicken is no longer pink (180 degrees F in thigh muscle). (For a gas grill, preheat grill. Reduce heat to medium. Place chicken, skin side down, directly over medium heat. Top with brick. Cover and grill chicken for about 8 minutes or until nicely browned. Remove brick; turn chicken. Adjust for indirect cooking. Place chicken on grill rack as directed over burner that is turned off. If necessary, remove upper grill racks. Grill as directed.)
Remove chicken from grill; let stand for 5 minutes before cutting into quarters with a knife.