Begin by heating olive oil in a large skillet over medium-high heat.
Add onions and salt. Cook, stirring frequently 20 minutes, or until light golden brown. Remove from heat and allow to cool.
Meanwhile, in a medium bowl, combine the sour cream, mayonnaise, cream cheese, garlic powder, black pepper, and cayenne. Use spoon or a hand mixer to blend well.
Fold in cooled onions.
Refrigerate at least 2 hours; preferably overnight. Garnish with scallions and serve chilled.