How to Make It
Heat oil in a large skillet over medium. Add onions to skillet, and cook, stirring often, until onions begin to caramelize, about 40 minutes. (You should have about 1 cup caramelized onions.)
Meanwhile, place potatoes in a large saucepan. Fill with cold water to cover by 2 inches. Add 2 tablespoons of the salt, and bring to a boil over high. Reduce heat to medium-high, and gently boil until potatoes are tender, about 12 minutes. Remove from heat, and drain. Return cooked potatoes to saucepan.
Mash potatoes to desired degree of smoothness. Stir in chicken broth, caramelized onions, pepper, and remaining 2 1/2 teaspoons salt. Transfer to a bowl or platter, and sprinkle with chives.