Smoky Barbecue Chicken With Balsamic Onions

Ingredients

• 1 medium red onion, trimmed, peeled and quartered
• 2 tbsp. balsamic vinegar
• 1 tsp. ground cumin
• 1 tsp. kosher salt
• 1⁄2 tsp. freshly ground black pepper
• 1⁄2 tsp. chipotle powder
• 2 1⁄2 pounds skinless chicken thighs, boneless
• 1 cup barbecue sauce, plus more for serving (optional)
• 2 tbsp. chicken stock
• Sandwich rolls or steamed rice