Pat the chicken thighs dry and trim away excess fat. Season both sides with kosher salt, black pepper, and smoked paprika.

In a bowl, stir together the prepared barbecue sauce, brown sugar, apple cider vinegar, Worcestershire sauce, and minced garlic until smooth.

For deeper flavor, heat the olive oil in a skillet and brown the seasoned thighs for 6–8 minutes, turning once. This step is optional, so you can place the seasoned chicken directly into the slow cooker if preferred.

Place the sliced onion in the slow cooker if using, then arrange the chicken thighs on top. Pour the barbecue mixture over the chicken and turn the pieces once so they are coated.

Cover and cook on LOW for 4–5 hours or HIGH for 2–3 hours. Check the thickest part with an instant-read thermometer; the chicken must reach at least 165°F, although boneless thighs are often especially tender around 175–190°F.

Transfer the cooked chicken to a plate and let it rest for about 5 minutes. For thicker sauce, simmer the cooking liquid in a saucepan for 5–10 minutes, or brush the chicken with additional barbecue sauce.

Spoon the sauce over the rested chicken and serve warm. For caramelized edges, brush the thighs with sauce and broil them for 2–3 minutes, watching closely.