Peel and thinly slice the onions.
Melt the butter in a skillet over medium heat and add the sliced onions. Stir occasionally as they cook, lowering the heat to medium-low once they start to brown. Cook for 20–25 minutes until golden and caramelized, stirring to prevent burning.
Let the caramelized onions cool for 5 minutes.
In a food processor, combine the cream cheese, sour cream, mayonnaise, salt, and pepper. Blend until smooth.
Add the cooled onions to the food processor and pulse a few times to mix them in while keeping some texture.
Transfer the dip to a bowl, garnish with chives, and serve with chips or veggies.