Step 1Lift up the skin on the cut side of the chicken breasts and insert the garlic slices. Distribute them evenly under the skin, taking care not to break the skin. Rub the sage leaves between your hands to release their oils and insert the leaves under the skin with the garlic cloves. Pull the skin back over the breast. If the chicken has not been brined, salt and pepper liberally on both sides. Wrap tightly in plastic and refrigerate for several hours or overnight.
Step 2Heat a large cast-iron skillet over high heat until hot. Add the olive oil and heat to just below smoking. Season the skin side of the chicken breasts with salt and pepper and place skin-side down in the pan. Season the other side of the breasts. Turn the heat down to medium low and place another heavy pan on top of the breasts, so that they are pressed flat into the pan. If the second pan is not heavy, weight it with canned tomatoes. Make sure the weight is evenly distributed over both breasts. They must cook evenly and at the same time, because once you turn them over you can no longer weight them. Cook until very brown and crisp, 12 to 15 minutes.
Step 3Turn the breasts over and cook for 5 more minutes, or until cooked through. Remove from the heat, let stand 5 minutes, then slice and serve, or cut each breast in half and serve.