Prep the chicken and onions: Pat the chicken thighs dry with paper towels and season both sides with 1 tsp kosher salt and 1/2 tsp pepper. Drying the meat ensures better browning (the Maillard reaction) when you sear it — that brown crust adds savory depth that a slow cooker alone can’t create. Thinly slice the onions; thinner slices caramelize and break down faster in the slow cooker.

Caramelize the onions: Heat a 12-inch skillet over medium heat and add the olive oil and butter. When the butter foams, add the sliced onions and a pinch of salt. Cook, stirring every 3–4 minutes, for 12–15 minutes until deep golden and sweet. This step blooms the onion’s natural sugars and builds a rich base flavor; raw or simply softened onions won’t give the same depth.

Add the sugar to the onions: Sprinkle the 2 tablespoons brown sugar over the onions in the pan and stir for 1–2 minutes until it dissolves and begins to coat the onions. Toasting the sugar briefly helps it integrate and create those sticky, caramelized notes that will flavor the sauce.

Sear the chicken (optional but recommended): Increase the skillet to medium-high. Push the onions to the side, add a tiny drizzle of oil if the pan is dry, and sear the chicken thighs 1–2 minutes per side until lightly golden (they don’t need to be cooked through). Searing develops additional flavor and improves the final look and mouthfeel of the chicken.

Deglaze and build the sauce: Pour the soy sauce, chicken broth and apple cider vinegar into the skillet to deglaze, scraping any browned bits up with a wooden spoon — these bits are packed with flavor. Stir in the minced garlic, smoked paprika, remaining salt and pepper. Allow the mixture to simmer 1–2 minutes off heat to marry flavors; the liquid will be the braising medium in the crock pot.

Arrange in the slow cooker: Transfer half the caramelized onions to the bottom of a 4–6 quart crock pot. Nestle the seared chicken thighs in a single layer on top, then spoon the remaining onions over the chicken and pour the deglazed pan sauce evenly over everything. Layering the onions under and over the chicken ensures every piece cooks in contact with the sweet, savory sauce.

Slow-cook: Cover and cook on LOW for 4 hours, or HIGH for 2½–3 hours. Low-and-slow yields the most tender, succulent meat because gradual collagen breakdown yields moist meat without drying. Avoid cooking much longer than directed — even thighs can dry or develop a stringy texture if overcooked in a thin sauce.

Check doneness and optional shredding: Chicken is done when it reaches 74°C (165°F) internal temperature and the pieces pull apart easily. For a rustic plating, leave thighs whole; for sandwiches or bowls, transfer chicken to a cutting board and shred with two forks — shredding lets the sauce penetrate more layers of meat.

Finish the sauce: Pour the crock pot juices into the skillet and bring to a simmer on medium-high. If you want a glossy, clingy sauce, whisk the cornstarch with 1 tablespoon cold water to make a slurry, then whisk into the simmering sauce and cook 1–2 minutes until thickened. Reducing concentrates flavor and the cornstarch gives a restaurant-style glaze that coats the chicken.

Coat and rest: Return the chicken to the skillet or spoon the thickened sauce over the pieces in the slow cooker. Let rest 5 minutes so the juices redistribute and the sauce sets slightly — this improves texture and sauciness when plated.

Serve: Garnish with chopped parsley and an extra crack of black pepper. Serve over mashed potatoes, creamy polenta, buttered noodles, steamed rice, or in a sandwich with toasted brioche for a restaurant-style presentation. The sticky-sweet onion glaze pairs especially well with creamy or starchy sides that balance the sauce.