Begin by patting the whole chicken dry with paper towels, inside and out. This is crucial for achieving that coveted crispy skin. In a small bowl, zest both lemons, then juice one of them. Finely chop the rosemary leaves and strip the thyme leaves from their stems. Mince 4 of the garlic cloves. Combine the lemon zest, lemon juice, chopped herbs, minced garlic, olive oil, kosher salt, and black pepper into a vibrant, fragrant paste. This marinade is the heart of the dish, so take a moment to inhale its bright, earthy aroma.

Generously rub the herb paste all over the chicken, ensuring it gets under the skin of the breasts and thighs for maximum flavor penetration. Place the chicken in a large bowl or a resealable bag, cover, and refrigerate for at least 2 hours, or ideally overnight. When ready to cook, preheat your oven to 425°F (220°C). Remove the chicken from the fridge 30 minutes before roasting. Stuff the cavity with the remaining lemon halves, the remaining 2 garlic cloves (smashed), and the rosemary and thyme sprigs. Tie the legs together with kitchen twine and tuck the wing tips under the body for even cooking.

Slice the onion into thick rounds and arrange them in the center of a large roasting pan or a heavy-duty ovenproof skillet. Place the trussed chicken directly on top of the onion slices, which will act as a natural rack. Pour the white wine and chicken broth into the pan around the chicken, being careful not to wet the skin. Roast the chicken for 45 minutes, then baste it with the pan juices. Continue roasting for another 20-30 minutes, or until a meat thermometer inserted into the thickest part of the thigh registers 165°F (74°C) and the skin is a deep, burnished gold.

Once the chicken is perfectly cooked, carefully tilt the bird to allow the juices from the cavity to flow into the pan. Transfer the chicken to a cutting board, tent it loosely with foil, and let it rest for at least 15 minutes. This step is non-negotiable for juicy meat. While it rests, place the roasting pan over medium heat on the stovetop. Add the butter and swirl to melt, scraping up the browned bits from the bottom. Simmer the sauce for 2-3 minutes until slightly thickened. Carve the chicken into beautiful pieces, arrange on a warm platter, and spoon the rich, lemony pan jus over the top. Serve immediately.