MELT butter in a large skillet over medium-low heat. Add onions and ½ teaspoon salt and cook, stirring occasionally, until onions are deeply golden brown, 30 to 35 minutes. Let cool. (If desired, roughly chop onions.)
MIX in a medium bowl the onions, cream cheese, mayonnaise, chives, ¼ teaspoon salt and ⅛ teaspoon pepper.
SERVE at room temperature, or cover and chill. Keeps well up to 2 days.