Add the onions, salt, pepper, onion powder, garlic powder, and cayenne pepper. Cook for 5 minutes, stirring.
Reduce the heat to medium-low and cook for 20 minutes, stirring occasionally, until the onions are sweet and golden. Remove from stove and allow to cool.
In a bowl, mix the cream cheese, sour cream, and mayonnaise until smooth. Add the caramelized onions and mix until just combined. Refrigerate for at least 2 hours to allow flavors to develop.
Sprinkle chopped scallions for garnish. Serve the dip at room temperature with your choice of chips or fresh vegetables.