The jam is meant to be served with Grilled Rack of Lamb, but is also delicious with the Fried Chicken with Cornmeal Crust from the same Montana Dinner Party menu.
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.