In a large skillet, heat the olive oil. Add the onions, season with salt and cook over moderately high heat, stirring, for 5 minutes. Reduce the heat to moderate and cook, stirring occasionally, until the onions are softened, about 10 minutes. Stir in the honey and balsamic vinegar and cook over low heat, stirring occasionally, until richly caramelized, about 10 minutes. Season with salt and pepper.