Barbecue Pork Belly Sandwich

Name: Barbecue Pork Belly Sandwich
Ingredients: 3 lb. pork belly, trimmed into a 5″ x 2 1/2″ rectangle; Kosher salt and freshly ground black pepper; 1 1⁄2 tbsp. vegetable oil; 2 medium carrots, roughly chopped; 2 stalks celery, roughly chopped; 1 bay leaf; 3–4 sprigs fresh thyme; 6 cups chicken stock; 1 cup ketchup; 1⁄4 cup water; 1⁄4 cup apple cider vinegar; 1⁄8 cup brown sugar; 1 1⁄2 tbsp. yellow mustard; 1 1⁄2 tbsp. onion powder; 1 1⁄2 tbsp. garlic powder; 1 tbsp. mesquite liquid smoke; 1 1⁄2 tsp. cayenne