Preheat oven to 350 degrees F. In a small bowl, combine sugar, salt, rosemary, and pepper flakes.
With a sharp knife, carefully cut a 1" grid pattern into the fat of the pork belly. Coat pork all over with rub.
In a large pot, heat oil over medium-high heat. Place pork belly into hot pan, scored fat side down. Cook for 2 to 3 minutes until well caramelized. Flip the pork over, and add beef stock, onion, and carrots. Place pot in oven, and cook uncovered for 1 1/2 hours.