For the sauce:
For the burgers:
Heat a grill to medium-high heat. To make the barbecue sauce, combine all the ingredients in a saucepan over medium heat. Cook, stirring occasionally, until the sauce thickens and is reduced to about 1 cup, about 7 minutes. Remove from the heat. Chill until ready to use.
To make the burgers, combine the ground chicken, cornmeal, scallion, cilantro, brown sugar, spices, pulled pork (if using) and 2 tablespoons of the barbecue sauce in a medium bowl and mix gently until well combined. Form the mixture into four equal patties. Transfer to a plate and refrigerate until ready to grill.
Oil the grates of the grill. Place the burger patties on the heated grill and cook about 3-4 minutes. Flip the burgers and brush the cooked sides generously with some of the remaining 3 tablespoons of barbecue sauce. Cook about 3 minutes, then flip once more and brush the second side with barbecue sauce. Top with cheese slices, if using. Cook just until cooked through and cheese is melted. Place the burgers on the toasted buns and top with grilled onion slices and additional barbecue sauce as desired.