In a 5 to 6-quart crock-pot, combine onions, garlic, broth, onion soup mix, cranberry sauce, and dried cranberries. Place brisket fat side up. Make sure to spread onions and sauce over brisket. Cover and cook on low heat until brisket is fork-tender, about 6 hours. At this point, cool slightly and let this dish refrigerate overnight. Keep meat in the sauce.
The next day: remove any excess fat that has formed on the top. Remove brisket and thinly slice the meat against the grain. To do this, secure meat with tongs or a fork and carve with a sharp knife. Slice downward on an angle with long strokes. Add sliced meat back to sauce and refrigerate.
To reheat: preheat oven to 350 degrees. Place crock-pot dish, covered with foil, in oven and heat for about 45-60 minutes until completely heated.
To serve: place slices of brisket on a serving plate. Pour sauce with onions all over meat. Serve remaining sauce on the side to pour over potatoes or rice. See serving tips below.