Preheat oven to 450°F. In a large mixing bowl combine potatoes, onion wedges, 2 tablespoons of the olive oil, rosemary, garlic, salt, and black pepper. Toss potato mixture to coat well; arrange in a single layer in a greased shallow roasting pan. Roast, uncovered, for 25 to 30 minutes or until potatoes are tender and lightly browned, stirring mixture twice. Transfer roasted potato mixture to a large bowl; set aside.
In a small bowl whisk together balsamic vinegar and remaining 2 tablespoons olive oil. Pour mixture over roasted potato mixture. Add red pepper pieces; toss gently to coat.
Turn potato salad into serving bowl. Sprinkle with pine nuts. Serve warm or at room temperature.
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