Rinse the chicken and pat it dry with paper towels. Drizzle the chicken (inside the cavity as well as exterior) with enough olive oil to lightly coat, and season with The Usual seasoning, or a generous amount of salt and pepper. Use your hands to distribute the spices evenly. Tuck the onion, lemon, garlic cloves, and herbs inside the cavity (whatever amount will fit in comfortably), and then use cooking twine to truss the chicken.
Place the rosemary sprigs in a large cast iron skillet, and then place the chicken, skin-side up, over the herbs and marinate at room temperature while you prepare the grill. (Alternately, the chicken can be marinated in a baking dish or in a sealable plastic bag for up to 12 hours in advance; remove from fridge an hour before grilling).
Prepare a charcoal grill for two-zone cooking and build a medium-high fire, or heat a gas grill to high. Clean and oil the grill grates. When the coals are glowing red and covered with a fine gray ash, place the skillet with chicken over indirect heat, close grill, vent for smoking, and cook for 15 to 20 minutes. Open the grill, carefully tilt the skillet and use a spoon to baste the chicken with the rendered juices. Rotate the skillet 180 degrees, close the grill, and continue roasting until the chicken is cooked through (when its juices run clear and/or when an instant read thermometer inserted between the leg and thigh reads 165°F), approximately 30-40 minutes longer. Baste the chicken and rotate the skillet every 15 minutes.
Remove from heat and let chicken rest for at least 10 minutes (or up to 30). While the chicken rests, grill the bread (if desired) over direct heat until lightly charred on both sides. Carve and serve the chicken (discarding rosemary sprigs) over toasted bread, with the delicious pan drippings spooned over the meat.