Pat the chicken thighs dry with paper towels and season both sides with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper. Drying and seasoning first ensures better browning during the sear and builds early flavor.

Heat a large skillet over medium-high heat and add the olive oil. When the oil shimmers, add the chicken thighs in a single layer (work in two batches if needed). Sear 2–3 minutes per side until deeply golden (don’t worry if they’re not cooked through). Searing creates Maillard browning — those caramelized bits add depth to the final sauce.

Remove the seared chicken to a plate and reduce heat to medium. Add the butter to the skillet. Once melted, add the sliced onions and a pinch of salt (about 1/2 tsp). Cook, stirring often, for 10–12 minutes until the onions are deep golden and beginning to caramelize. Stirring and moderate heat let the onions develop sweetness without burning.

Add the minced garlic and the brown sugar to the onions and cook 1–2 minutes more, stirring. The sugar will help the onions glaze and build a shiny, rich base for the sauce; cook briefly to avoid burning the sugar.

Pour in the chicken broth and apple cider vinegar to deglaze the pan, scraping up any browned bits (fond) from the bottom with a wooden spoon. Deglazing lifts concentrated flavor into the liquid, which becomes the base of the slow-cooker sauce.

Transfer the caramelized onion mixture and any pan juices into the slow cooker. Nestle the seared chicken thighs into the onions. Tuck the thyme sprigs around the chicken. Sprinkle the remaining 1/2 tsp salt and 1/2 tsp pepper over the top. Positioning builds layers of flavor — the chicken will steam-braise in the onion-scented broth.

Cover and cook on LOW for 3 to 3 1/2 hours, or on HIGH for 1 1/2 to 2 hours, until the chicken is tender and an instant-read thermometer reads 74°C (165°F) where the meat is thickest. Slow, gentle heat breaks down connective tissue and yields tender meat; don’t overcook on HIGH or it can dry out.

When the chicken is done, remove it to a warm platter and tent loosely with foil to rest for 5 minutes. Remove and discard thyme sprigs. Resting lets the juices redistribute so the meat stays moist.

If you prefer a thicker glaze, skim pan juices into a small saucepan. Mix 1 tsp cornstarch with 1 tbsp cold water to make a slurry, bring the juices to a simmer and whisk in the slurry until glossy and slightly thickened (30–60 seconds). Taste and adjust seasoning. Thickening the sauce concentrates its flavor so it clings to the chicken.

Spoon the caramelized onion glaze over the chicken, garnish with fresh thyme leaves if desired, and serve hot. The glossy onion glaze and tender braised chicken create a sweet-savory contrast that’s the hallmark of the dish.