Finely chop garlic on a cutting board. Sprinkle the salt and the chopped sage on top, then finely chop again. Pulverize with the flat part of a chefs knife to make a paste.
Transfer the garlic-sage paste to a small bowl. Juice in ONE of the lemons. Add the ground pepper and the olive oil. Mix well to combine the marinade.
Rub the marinade all over the chicken until thoroughly coated. Loosen the skin around the neck opening and rub a little marinade directly on the breasts.
Cover loosely with plastic wrap and refrigerate for at least 4 hours and up to overnight.
Preheat a charcoal grill to 450F with both direct heat over coals and indirect heat. Bring the chicken out of the fridge while the coals are heating up.
With a sturdy pair of tongs, place the chicken, breast side down, onto the grate and grill directly over the coals. Grill for 5-6 minutes, or until the skin is crispy and slightly charred. Extinguish any flames with a spray bottle. Flip the bird and grill for an additional 5 minutes. The bird should be lightly charred all over.
Slice the remaining lemon in half and add it to the grill, cut side down. Move the chicken off the coals, over to indirect heat. Close the lid of the grill and cook the chicken for about 15-20 minutes. The legs can indicate doneness, as per my Notes below.
Remove the chicken and lemon from the grill and place on a serving platter. Tent loosely with foil. Allow the bird to rest for 10 minutes before carving. (This is a good time to grill your side dish)
Squeeze the grilled lemon all over the chicken, garnish with a few more sprigs of sage and serve.
Notes
So how can you tell when your chicken is cooked? Pay attention to the legs and they will tell you everything you need to know:- Juices run yellow, not pink, when drumstick is pierced. - The drumstick wiggles freely at the joint when manipulated. A meat thermometer registers 170°F when inserted into the thickest part of the thigh (but not touching the bone).