Prep the chicken: Pat the chicken dry with paper towels.
This helps it brown well. Season all sides with salt and black pepper.
Make the rosemary-garlic mix: In a small bowl, combine the chopped rosemary, minced garlic, lemon zest, 1 tablespoon lemon juice, olive oil, and red pepper flakes if using.
Coat the chicken: Rub the mixture over the chicken, getting it under the skin if using skin-on thighs. Let it sit while you heat the pan.
Even 10 minutes makes a difference.
Sear for color: Heat a large oven-safe skillet over medium-high. Add a light slick of olive oil. Place chicken skin-side down and sear without moving until deep golden, about 4–6 minutes.
Flip and sear the other side for 2–3 minutes. Work in batches if needed.
Build the pan base: Reduce heat to medium. If using, add sliced onion or shallot to the skillet and cook 2–3 minutes, stirring.
Stir in the butter for a silky sauce.
Deglaze: Pour in the chicken broth or wine and the remaining lemon juice. Scrape up browned bits with a wooden spoon. Nestle a few rosemary sprigs around the chicken.
Roast to finish: Transfer the skillet to a preheated 400°F (200°C) oven.
Roast until the chicken reaches 165°F (74°C) at the thickest part. Bone-in thighs: about 18–22 minutes after searing. Boneless thighs: 12–15 minutes.
Breasts: start checking at 12 minutes.
Rest and spoon juices: Let the chicken rest 5 minutes. Spoon the garlicky, lemony pan juices over the top. Taste the sauce and adjust salt and lemon as needed.
Serve: Pair with roasted potatoes, buttered noodles, rice, or a simple green salad.
Don’t waste the sauce—serve with bread if nothing else.