Preheat oven to 400°F Coat a rimmed baking sheet with cooking spray.
Season both sides of chicken with salt and pepper and arrange on baking sheet. Sprinkle with Creole seasoning and bake for 25-30 minutes, until chicken is just cooked through.
Meanwhile, make the onions. Heat oil in a large sskillet over medium high heat. Add the onions and brown sugar and cook until onions are soft and golden brown, about 7 minutes. Add the rum and vinegar and simmer for 3-5 minutes, until liquid is absorbed.