Thinly slice the onions. Heat oil in a large skillet over medium heat. Add onions and cook, stirring frequently, until caramelized, approx. 20 minutes. Transfer onions to a small bowl and place in fridge for approx. 1 hour- until chilled.
In a food processor, pulse onions, sour cream, buttermilk, and salt just until combined. Transfer to a serving dish, and chill at least 1 hour.