In a medium saucepan, bring potatoes and enough water to cover by 1 inch to a boil; add salt. Cover, reduce heat and simmer until tender, 15 to 20 minutes.
Meanwhile, in a small skillet, heat the oil over medium-low heat. Add onions and season with salt. Cook the onions, stirring frequently, until caramelized, 15-20 minutes.
Drain the potatoes and return to the pan. Add the onions with all of the oil. Mash until smooth. Season with freshly ground black pepper. Serve immediately.
~ Free Tip ~ For the fluffiest, smoothest mash choose potatoes high in starch, like Russet or Yukon Gold. If you like your potatoes smashed (with chunks) you can use Idaho potatoes. For even more flavor, sauté minced garlic clove with the onions.