Melt the butter in a heavy-bottom pot or Dutch oven over medium heat. Add in the onions. Cook, stirring occasionally, until the onions are tender and mostly brown—about 20 minutes. Remove from heat.
Using an electric mixer, blend the cream cheese until creamy and smooth. Add in the yogurt, mayo and Worcestershire, and blend until smooth. Add in the onions, chives, salt and pepper. Transfer to an airtight container, and refrigerate for at least two hours before serving (preferably overnight for best flavor).
Notes
To keep this recipe gluten-free, be sure to use a gluten-free Worcestershire sauce!