Prepare and season the chicken: Pat the thighs very dry with paper towels (dry skin equals better browning). Season both sides generously with 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Let sit at room temperature for 10 minutes while you preheat — warming the meat slightly encourages even cooking.

Preheat oven and skillet: Place a heavy ovenproof skillet (cast-iron preferred) on the stovetop and preheat the oven to 400°F (200°C). Heat the skillet over medium-high heat until hot — a properly preheated pan helps crisp the skin and achieve a strong Maillard reaction.

Sear the thighs skin-side down: Add 1 tablespoon olive oil plus 2 tablespoons of butter to the hot skillet. When the butter stops foaming and the fat shimmers, carefully lay the chicken thighs skin-side down in a single layer. Press each thigh lightly with a spatula for 10–15 seconds to ensure full contact. Sear without moving for 7–9 minutes until the skin is deeply golden-brown and crisp. Searing creates flavorful browned compounds and renders some of the fat from the skin.

Flip and brown the underside: Turn the thighs over and sear the flesh side for 2 minutes to color the meat. This short sear locks in juices and adds flavor on both sides.

Add aromatics before roasting: Reduce heat to medium. Push the chicken to make a small well in the pan and add the remaining 1 tablespoon olive oil if the pan looks dry, then add the sliced onion, smashed garlic, and 4 thyme sprigs. Season the aromatics with the remaining 1/2 teaspoon salt and 1/2 teaspoon black pepper. The aromatics will release flavor into the pan fat and imbue the chicken as it roasts.

Finish in the oven: Spoon 2 tablespoons of the butter over the thighs (basting butter helps keep skin crisp while flavoring the meat) and transfer the skillet to the preheated oven. Roast for 18–22 minutes until the internal temperature reaches 165°F (74°C) at the thickest part of the thigh (avoid touching bone). Oven finishing cooks the thighs evenly through while preserving the crisp skin from the sear.

Rest the chicken: Remove the skillet from the oven and transfer the thighs to a warm plate. Tent loosely with foil and rest for 8–10 minutes. Resting allows juices to redistribute so the meat stays moist when you cut into it.

Make the brown butter garlic pan sauce: Return the skillet to medium heat on the stovetop (use the pan juices and browned bits left behind — they are concentrated flavor). Add the remaining 4 tablespoons butter to the pan. Let it melt and foam, swirling occasionally; watch closely as the milk solids will begin to brown and smell nutty in about 2–3 minutes. This browning transforms plain butter into a complex, toasty sauce base. As the butter turns golden-brown, stir in the chopped garlic and cook briefly (30–45 seconds) — you want fragrant garlic but not burned garlic.

Deglaze and reduce: Pour in 1/4 cup water (or low-sodium chicken stock if you have it) to deglaze the pan, scraping up all the browned bits with a wooden spoon — those bits are the backbone of the sauce. Bring to a simmer and reduce the liquid by half, which concentrates flavor and gives the sauce a glossy texture.

Finish the sauce and adjust seasoning: Taste the sauce and adjust with a pinch of salt and a grind of black pepper as needed. Remove and discard the thyme sprigs if the stems are woody. Spoon the sauce over the rested chicken thighs just before serving to preserve crisp skin.

Serve: Arrange the thighs on a warmed platter, spoon a little of the brown butter garlic sauce and pan onions over each piece, and serve immediately. The contrast between crisp skin, juicy meat, and silky browned-butter sauce is the hallmark of the dish.