Melt butter in a large skillet over medium heat and add the sliced onions, salt, and pepper.
Cook the onions slowly, stirring occasionally, for 30-35 minutes until deeply caramelized and golden brown.
Add the thyme and garlic during the last 2 minutes of cooking, then remove from heat and let cool completely.
In a medium bowl, combine the sour cream, mayonnaise, and Worcestershire sauce.
Fold the cooled caramelized onions into the sour cream mixture until well combined.
Cover and refrigerate for at least 2 hours or overnight before serving to allow flavors to meld.