In a large skillet, heat the olive oil over medium heat.

Add the chopped onions and salt, cooking until the onions are caramelized and golden brown, about 15 minutes.

Once the onions are caramelized, add black pepper and Worcestershire sauce, stirring well to combine.

In a mixing bowl, combine the sour cream and mayonnaise until smooth.

Add the grated Swiss cheese and chopped chives to the sour cream mixture.

Fold the caramelized onions into the creamy mixture.

Transfer the onion dip mixture to an oven-safe baking dish, spreading it evenly.

Bake in a preheated oven at 350°F for about 30 minutes.

Remove from the oven and let it cool slightly before serving.