If the chicken breasts are thick, pound them to an even thickness of about 3/4 inch. Pat them dry, then season both sides with the kosher salt and freshly ground black pepper.

Heat the olive oil in a large skillet over medium-high heat. Add the chicken and cook for about 5–6 minutes per side, depending on thickness, until browned and the internal temperature reaches 165°F.

Transfer the cooked chicken to a plate and loosely cover it. Reduce the skillet heat to medium.

Add the minced garlic and chopped rosemary to the skillet. Cook for about 30 seconds, just until fragrant, taking care not to let the garlic brown.

Add the lemon zest, lemon juice, and chicken broth. Scrape the browned bits from the bottom of the skillet as the liquid begins to simmer.

Simmer the sauce for 2–4 minutes, until it is slightly reduced. Whisk in the optional unsalted butter for a smooth, glossy finish.

Return the chicken to the skillet and spoon the sauce over the breasts. Simmer for 1–2 minutes, then serve hot with the pan sauce.