2 to 4 bricks wrapped in aluminum foil, or metal grill presses
Place the garlic, 2 tablespoons each of chopped rosemary and sage, and the pepper in a food processor fitted with a metal chopping blade and run the machine in short bursts to finely chop. Add the salt and process to mix. Transfer the rub to a jar with a tight-fitting lid. This makes about 3/4 cup of herbed salt rub, more than you’ll need for this recipe. The leftover rub is also excellent for seasoning veal, pork, and pheasant and will keep for several weeks in the refrigerator.
Remove and discard the fat just inside the neck and body cavities of the chickens. Remove the packages of giblets and set them aside for another use. Rinse the chickens, inside and out, under cold running water, then drain and blot them dry, inside and out, with paper towels. Spatchcock the chickens. Generously season the birds on both sides with the herbed salt rub.