In a large pot or dutch oven, cook the bacon, working in batches if neccessary. Remove the bacon from the pan and allow to cool and then roughly chop. Drain off the bacon grease from the pot, leaving only about 1 tbs bacon drippings in the bottom of the pot. Return the pot to heat and cook the onions until soft and translucent, about 3 minutes. Add the garlic and stir for about 30 seconds. Add 1 cup beer and both vinegars, scraping to deglaze the bottom of the pot. Add the brown sugar and the bacon, reduce heat to maintain a simmer. Place the lid on the pot at an angle, allowing to vent the steam. Cook until reduced to a thick and syrupy consistency, stirring occasionally, about 45 minutes. Transfer to a food processor along with remaining 1/4 cup beer and pulse until most of the large pieces have been chopped.