Pour the olive oil into a large skillet and place over high heat. Add the onions and garlic and cook, stirring to coat in oil. Reduce the heat to medium, add the balsamic, sugar, salt, and red pepper flakes and cook 15 to 20 minutes, until the onions are velvety soft. remove from the heat and let cool and add the chopped chives.
Serve at room temperature in a decorative bowl.