Place potatoes in a large pot of salted water. Bring to a boil and cook 25 minutes or until tender. Drain and cool completely.
Meanwhile, cook onions in oil in a sauté pan over medium heat for 20 minutes or until caramelized. Stir occasionally.
Cut potatoes in half and scoop out pulp with a melon baller, leaving a sturdy wall. Mash potato pulp and fold in sour cream, salt and pepper. Fill potatoes with mixture and top with onions. Serve room temperature.
Note: To serve hot, place in 350-degree oven for 10 minutes and serve immediately.