Cut the onions in half and then slice them into 1/8-inch thick half-rounds. (You will have about 3 cups of onions.) Heat the butter and oil in a large saute pan over medium heat. Add the onions, cayenne, salt, and pepper and saute for 10 minutes. Reduce the heat to medium-low and cook, stirring occasionally, for 20 more minutes until the onions are browned and caramelized. Allow the onions to cool.
Place the cream cheese, sour cream and mayonnaise in the bowl of an electric mixer fitted with the paddle attachment and beat until smooth. Add the onions and mix well. Taste for seasonings. Serve at room temperature.
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