In a large skillet over medium heat, combine the olive oil and butter. Add the onions and sauté for 10 minutes, stirring occasionally. Reduce heat to medium-low and cook for an additional 20 minutes until onions are deep golden and caramelized. Add 2 tablespoons of water and stir, scraping the browned bits off of the bottom of the pan. Cook until the water is gone. Season with a pinch of salt and pepper and cool.
With a stand mixer and a paddle attachment or a hand mixer, cream together the cream cheese, sour cream and mayo. Add the kosher salt, onions and chives and mix to combine. Taste and add additional salt and pepper, if needed.