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, preheat the oven to 350F. 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 slice onions until deeply browned. Pour in half cup of coffee. Bring to the boil, stirring and scraping the bottom of the skillet to loosen the browned bits. Spread the onions and the 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 the oven for half an hour. Turn down the oven to 250F and bake for an additional 2 hours or until meat is very tender.
Slice the brisket 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.View more Beef recipes...