In sauté pan set over medium heat, melt butter. Add onions, salt and pepper. Cook until caramelized, about 45 minutes, stirring occasionally as needed. If onions begin to burn, turn down heat. Remove from heat and let cool to room temperature.
In food processor, pulse onions, cream cheese and sour cream until smooth. Taste and add salt and pepper to taste.
Put dip into bowl and serve right away or chill until ready to serve. Dip will keep in the fridge for about a week.
Top with chives before serving, if desired.