Short ribs, also called flanken, make their own rich, delicious sauce as they cook. The meat should be soft and melting when done.
Place flour in a shallow bowl. Season ribs with salt and pepper, then dredge in flour, shaking off excess. Heat 1 tablespoon chicken fat in a large Dutch oven over medium-high heat until hot but not smoking. Add half the ribs (do not crowd) and half the rosemary. Brown ribs very well on all sides, five to seven minutes total. Transfer to a plate. Repeat with remaining ribs, tablespoon fat, and rosemary.
Add onions, carrots, and celery; season with salt and pepper. Cook over medium heat, stirring frequently, until golden brown, about 15 minutes.
Add wine, and bring to a boil; deglaze pot, scraping up browned bits from bottom with a wooden spoon. Add stock and Worcestershire sauce; return to a boil. Return ribs to pot; add just enough water to cover, if necessary. Cover pot; simmer over very low heat until meat is very tender and falling off the bone, about two hours. Let cool completely. Refrigerate, covered, until ready to proceed.
About one hour before serving, remove the hardened layer of fat. If desired, remove the bones; return meat to pot. Simmer until the meat is heated through, about 20 minutes. With a slotted spoon, transfer meat to a plate.
Raise heat to medium-high; reduce liquid, stirring occasionally, until slightly thickened, 20 to 25 minutes. Return meat to pot; cook for a few minutes to reheat. Garnish with rosemary; serve with horseradish on the side, if desired.
Each rib should be about five inches long and one and a half inches thick, with three short bones. You can remove the bones for a neater presentation or leave them in (the meat should be falling from them anyway).