Preheat your oven to 220°C (430°F) and set a roasting pan nearby. Pat the chicken dry with paper towels, paying special attention to removing excess moisture from the skin—this helps it turn crispy during roasting.
Generously season the chicken all over with kosher salt and freshly cracked black pepper, both inside the cavity and on the skin. This step ensures flavorful, well-seasoned meat and crispy skin.
Slice one lemon in half and stuff the halves into the chicken's cavity along with a few sprigs of thyme and rosemary. This infuses the meat with bright citrus and herbal aroma as it roasts.
Zest both lemons and rub the zest all over the outside of the chicken, then drizzle with olive oil and rub it in to promote crispness. Toss in the smashed garlic cloves and, if using, add onion slices underneath the chicken for added moisture and sweetness.
Tie the chicken legs together with kitchen twine to help it cook evenly and retain stuffing. Place the chicken breast-side up in the roasting pan, and put it into the oven. Roast at 220°C (430°F) for 15 minutes to start browning the skin.
Lower the oven temperature to 180°C (350°F) and continue roasting for about 45 to 60 minutes. Baste the chicken with pan juices halfway through to keep the skin golden and flavorful. Keep an eye on the skin, tenting with foil if it browns too quickly.
Check for doneness by inserting a meat thermometer into the thickest part of the thigh; it should read 75°C (165°F). The skin should be deeply golden and crispy, with aromatic lemon and herbal scents filling the air.
Once cooked, remove the chicken from the oven and transfer it to a cutting board. Cover loosely with foil and let it rest for 10–15 minutes to allow the juices to redistribute, ensuring tender, moist meat.
Carve the chicken into pieces, and serve with the pan juices spooned over the top. Garnish with additional lemon slices and fresh herbs if desired, for an extra burst of brightness.
