Slice all of the onions very thin. Heat the butter and oil in a large pan over medium heat. Add the onions, salt and pepper. Saute for 10 minutes, until the onions start to soften.

Reduce heat to medium low, and continue cooking for about 30-40 minutes, stirring frequently. 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. If the onions are not getting browned, you can add 1 teaspoon of sugar after 25 minutes.

In bowl of an electric mixer combine cream cheese, sour cream and mayo. Beat until smooth.

Add in the onions and mix until well combined.

Serve immediately or chill to serve later.