In a large pan, melt the butter over medium heat. Add the onions and sauté for 5 minutes then reduce the heat to medium-low. Continue to cook, stirring occasionally, until the onions are fully caramelized, about 1 hour.
When there are about 10 minutes of cook time left, add the minced garlic to the onions.
Remove the onions from the pan and allow to cool for 20 minutes.
In a bowl, stir together the sour cream, mayo, onions, parsley, chives, salt, and pepper until well combined.
Refrigerate for at least 2 hours, but ideally overnight.