In a large heavy-bottomed stockpot or sauté pan, melt the butter over medium-high heat. Add the onions and sauté, stirring every 2-3 minutes for about 30 minutes, or until the onions are caramelized to a deep golden brown color. (If the bottom of the pan starts to burn, just add in a tablespoon or so of water at a time.)
Add garlic and sauté for 1-2 minutes, until fragrant. Remove from heat and transfer the onion mixture to a medium mixing bowl. Refrigerate for 15-20 minutes or until the onions reach room temperature.
Set aside a few tablespoons of the onion mixture, which we will use later as a topping. Add the remaining ingredients to the bowl of caramelized onions, and stir until evenly combined.
Taste and season with additional salt and pepper and extra Worcestershire, if need be.
Serve immediately, topped with the reserved caramelized onions and extra chives, if desired. Or cover and refrigerate for up to 3 days.