With a long thin, sharp knife, make slits in the meat and insert the slivers of garlic. Place the meat in a bowl, spread 1 sliced onion and the crushed garlic over the meat, and pour in the vinegar. Marinate for 6 hours at room temperature or overnight in the refrigerator, turning several times. When ready to cook, prehaeat the oven to 350 degrees F. Heat the bacon fat in a deep, heavy skillet large enough to hold the brisket. Remove the brisket from the marinade and discard the onion and vinegar. Dry with paper towels. Brown the meat well on all sides. Remove brisket to a platter. In the fat remaining in the skillet, saute the remaining sliced onions until deeply browned. Pour in 1/2 cup coffee. Bring to a boil, stirring and scraping the bottom of the skillet to loosen the browned bits. Spread the onions and liquid from the skillet in a shallow baking dish. Place the brisket on the onions. Season with salt and freshly ground pepper to taste. Pour in the remaining coffee and water. Cover tightly with foil and place in oven for 1/2 hour. Turn the oven down to 250 degrees F. and bake for an additional 2 hours or until meat is very tender. Slice the brisket thinly against the grain. Skim the fat from the pan liquid. Return the meat slices to the pan. Serve at once or refrigerate for later use.