In a large heavy-bottomed stockpot, melt the butter over medium-high heat.  Add the onions and saute until well browned and caramelized (but not burnt), about 30-40 minutes, stirring every few minutes.  Add garlic and sauté for 1-2 minutes, until fragrant.  Remove from heat and transfer the onions to a medium mixing bowl.  Refrigerate for 15-20 minutes or until the onions reach room temperature.  (Optional: At this point, set aside 1/4 cup caramelized onions, if you would like to use them as a topping.)

Add the remaining ingredients to the mixing bowl with the caramelized onions, and stir until the mixture is evenly combined.  Taste, and season with additional salt and pepper and extra Worcestershire if need be.

Serve immediately, sprinkled with chopped chives if desired.  Or cover and refrigerate for up to 3 days.