Slice the bacon into 1 inch chunks and cook in a large skillet until crispy. Reserve the grease rendered from the bacon in a heat safe bowl- set aside. Drain the remaining grease from the bacon on a plate lined with a paper towel and set aside.
Slice your onions very thin and then give them a rough chop, so you do not have long strands of onion remaining. See photo for reference
In the emptied skillet add the onions, 3 tablespoons reserved bacon grease, and garlic. Saute over low heat until the onions are very soft and translucent, about 15 minutes.
Meanwhile, rough chop the cooled bacon.
Add all the bacon and 1/3 cup brown sugar to the onions, stir to coat and cook for 2 minutes.
Add the 2 tbsp raisins, 1 tbsp worcestershire, 3 tbsp vinegar and 1/3 cup water to the bacon mixture. Simmer over medium low heat for 5-8 minutes, until the sauce has thickened and looks syrupy.
Serve immediately or store in an airtight container in the fridge for 5-7 days.