Melt the butter in a large skillet over medium-low heat, then add the sliced onions and 1/4 teaspoon of the salt.

Cook the onions, stirring every few minutes, for 20 to 25 minutes, until deep golden brown and jammy rather than just soft; lower the heat if they start to catch at the edges.

Spread the caramelized onions on a plate and let them cool completely, about 15 minutes, so they do not melt the cream cheese.

Beat the softened cream cheese, sour cream and mayonnaise together with a hand mixer on medium speed for 1 minute, until smooth.

Stir in the cooled onions, Worcestershire sauce, remaining 1/4 teaspoon salt and black pepper, until evenly combined.

Spoon the dip into a serving bowl and smooth the top.

Chill for 30 minutes, until the dip firms up enough that a chip can be dragged through it without the dip collapsing around the edges.

Scatter the chopped chives over the top just before serving, and serve cold with ridged kettle chips.