1. Heat oil in a medium deep straight-sided skillet over medium-high heat. Add onions and cook, stirring occasionally, until softened and golden in places, about 5 minutes. Add sugar and cook, stirring, until sugar begins to caramelize, about 5 minutes more. Carefully add vinegar (mixture will splatter) and bring to a boil. Boil until syrupy and reduced by three quarters, about 8 minutes. Transfer to a bowl, season with salt and pepper, and let cool at least 30 minutes. Fold mint leaves into jam just before serving.