In a large sauté pan over medium / low heat add the butter and onions.
Once the onions begin to sweat and turn translucent add the thinly sliced garlic cloves.
Simmer over medium / low heat until the onions and garlic begin to break down and brown.
If you see the bottom of the pan begin to get crispy pieces simply deglaze the pan using some chicken stock. I had to deglaze my pan 3 or 4 times before the onions were fully caramelized.
Right before you take the onions and garlic off of the heat add your Worcestershire sauce. Let it simmer for about 5 minutes with the rest of the ingredients.
Turn off heat and set the caramelized onions and garlic aside.