Preheat and prep. Heat the oven to 450°F (232°C). Place a rack in the lower-middle position. Pat the chicken very dry with paper towels, inside and out.
Dry skin equals crisp skin.
Mix the herb butter. In a small bowl, combine softened butter, lemon zest, minced garlic, rosemary, thyme, parsley, salt, and pepper. Stir until evenly blended into a fragrant paste.
Loosen the skin. Using your fingers, gently separate the skin from the breast meat starting at the cavity end. Be careful not to tear the skin.
Butter under the skin. Slide about half the herb butter under the skin, smoothing it over the breasts.
Rub the remaining butter over the outside of the chicken, including the legs and thighs. Drizzle with olive oil for extra browning.
Season the cavity. Sprinkle a pinch of salt inside the cavity. Stuff with the halved lemons and the onion quarters if using.
This adds moisture and citrusy aroma.
Truss or tuck. Tie the legs together with kitchen twine, or at least tuck the wing tips under the body. Trussing helps the chicken cook evenly and keeps the aromatics in place.
Prepare the pan. Scatter potatoes and/or carrots in a roasting pan if using, season with salt and pepper, and drizzle with olive oil. Set a rack over the vegetables if you have one, or place the chicken directly on top so the drippings baste them.
Roast hot, then reduce. Place the chicken breast-side up and roast at 450°F (232°C) for 15–20 minutes to set the skin. Then reduce the oven to 375°F (190°C) and continue roasting for 45–60 minutes, depending on the size of your bird.
Check for doneness. The thickest part of the thigh should reach 165°F (74°C) on an instant-read thermometer, and juices should run clear.
If the skin is browning too fast, tent loosely with foil.
Rest. Transfer the chicken to a cutting board and rest for 15 minutes. This allows the juices to redistribute for moist, tender slices.
Make a quick pan sauce (optional). Place the roasting pan over medium heat on the stove. Skim excess fat.
Add 1/2 cup broth or white wine, scraping up browned bits. Simmer 3–5 minutes until slightly reduced. Squeeze in some roasted lemon juice and adjust seasoning.
Carve and serve. Carve the chicken into breasts, thighs, drumsticks, and wings.
Spoon over pan juices, sprinkle with fresh parsley, and serve with the roasted vegetables.