1. Bring potatoes, 1 tsp. salt, and water to cover to a boil in a large saucepan over high heat. Reduce heat to medium, and simmer 5 to 7 minutes or until potatoes are just tender. Drain.
2. Heat 1 side of grill to 300° to 350° (medium) heat; leave other side unlit. Remove and discard giblets and neck from chicken. Place chicken, breast side down, on a cutting board. Cut chicken, using kitchen shears, along both sides of backbone, separating backbone from chicken; discard backbone. Open chicken as you would a book. Turn chicken, breast side up, and press firmly against breastbone with the heel of your hand until bone cracks. Tuck wing tips under. Rub with 1 Tbsp. oil. Sprinkle chicken with black pepper and remaining 2 tsp. salt
3. Heat a 12-inch cast-iron skillet on lit side of grill 10 minutes. Add remaining 1 Tbsp. oil to hot skillet; place potatoes, cut side down, in skillet. Transfer skillet to unlit side of grill.
4. Place chicken, breast side down, over lit side of grill; top with herbs. Place cast-iron skillet with potatoes on chicken to flatten. Grill, covered with grill lid, 10 to 15 minutes or until chicken is browned. Remove skillet. Transfer chicken to unlit side of grill. Place cast-iron skillet on chicken. Grill, covered with grill lid, 45 minutes or until a meat thermometer inserted in thickest portion of breast registers 165°. Remove chicken from grill, and let stand 5 minutes. (For crisper skin, place chicken on lit side of grill, and grill, without grill lid, 5 minutes or until crisp.)
5. Meanwhile, place lemons, cut side down, on lit side of grill, and grill, covered with grill lid, about 5 minutes or until charred and softened. Serve chicken with potatoes and charred lemon.