1. Preheat oven to 450 degrees F. Using kitchen shears, butterfly the chicken by cutting along either side of the backbone and removing the bone. Place the chicken breast side up and press down firmly (you may hear a snap) until it lies flat. Place it in a baking pan, breast side up.
2. Thinly slice 2 lemons. Lift the skin up from the breast and work half of the lemon slices under the skin along the breast meat, along with the garlic and rosemary. Lay the remaining lemon slices on top of the chicken.
3. Slice the remaining lemon in half. Squeeze the juice of both halves over the chicken.
4. Roast the chicken for 10 minutes. Reduce heat to 400 degrees F and continue roasting until internal temperature of the thickest part of the breast reaches a minimum of 165 degrees F (30-35 minutes).
5. Tent with foil and let rest 5-10 minutes.