Cut the onions in half, and slice them into 1/4-inch thick half-rounds. This will make about 3 cups of onions.
Heat the butter and oil in a large sauté pan on medium heat.
Add the onions, cayenne, salt, and pepper and sauté for 10 minutes. Reduce the heat to medium-low and cook, stirring occasionally, for 20 minutes, until the onions are browned and caramelized. Allow onions to cool.
Place the cream cheese, sour cream and mayonnaise in the bowl of an electric mixer fitted with a paddle attachment and beat until smooth. Add the onions and mix well.
Taste for seasonings, serve at room temperature, and enjoy!