Heat the butter and oil in a large pan over medium heat. Add the onions, salt and pepper. Saute for 10 minutes. Reduce heat to medium low, and continue cooking for about 30-45 minutes. You want the onions to get soft and caramel in color. You don't want them browned from too high of a temp.
Once cooked, allow to cool slightly.
In bowl of an electric mixer combine cream cheese, sour cream and mayo. Beat until smooth and creamy.
Add in the onions and roasted garlic, mix until well combined.