Cut potatoes in half and then cut each half into 3 equal sized wedges. Cut onions in half lengthwise and then cut each half into 3 equal sized wedges.

In a large bowl toss potatoes, onions, salt, pepper, sage, oil and butter. Spread mixture in a greased shallow roasting pan (a jelly roll pan is perfect). Make sure that mixture is in one layer. Roast about 25 minutes or until potatoes are beginning to brown. Add vinegar to vegetables and stir well to combine.

Place back in oven and roast another 8 minutes or until vinegar becomes bubbly and slightly thickened. Let cool slightly and remove to a serving dish. Sprinkle with parsley. Serve warm or at room temperature.