In a bowl, combine the flour and broth until smooth. Stir in the cranberry sauce, cranberry juice, garlic and rosemary. Pour into a large roasting pan. Top with onion slices. Season the brisket with salt and pepper; place fat side up in he pan. Cover and bake at 180°C for 3 to 3-1/2 hours or until meat is tender, basting occasionally.
Remove brisket to a serving platter and let stand for 15 minutes. Thinly slice meat across the grain; serve with onion and pan juices.