Meanwhile, cut root end off onion, creating flat surface. Place flat surface on cutting board; cut onion in half vertically, and peel off outer layer. Place large flat side down; cut into 1/8-inch slices. In 10-inch skillet, heat oil over medium heat. Add onion and rosemary; cook 8 to 10 minutes, stirring frequently, until onions are caramelized. Stir in brown sugar.