Pre-heat oven to 325. In a large, heavy-bottom pot such as a Dutch oven, heat the oil on medium. Working in batches so as not to crowd the pan, sear the short ribs for 3 to 4 minutes on each side, or until well browned. Transfer the short ribs to a plate.
Add the leeks, onions, carrots, celery and garlic to the pot and cook until well browned, stirring occasionally, about 12 to 15 minutes. Add the pepper flakes, pepper and tomato paste, then cook until the tomato paste turns a brick-reddish-brown color, about 6 to 7 minutes.
Add the Dijon mustard, rosemary sprigs, balsamic vinegar and brown sugar. Scrape up any browned bits from the bottom of the pot. Return the short ribs to the pot, then add the beef stock and wine. Bring the mixture up to a low simmer and cover.
Cover braise with parchment paper just touching the rib put cover on pot. Place in a 325-degree oven for 2 - 3 1/2 hours, or until the short ribs are very tender when pierced with a fork.
Transfer meat to a platter. Cover with foil and a couple of kitchen towels to keep warm. Put mesh strainer over bowl, spoon remaining liquid into strainer. Discard the solids from the liquid. Put liquid in fat separater. Pour off liqiud to small sauce pan leaving fat behind. Bring the liquid to a boil on the stovetop and cook until reduced to 1 cup. Pull (shred) beef picking out extra fat. Meanwhile make polenta
Added polenta to bottom of pasta bowl or dish. Top with shreded beef. Drizzle the glaze over the shreded beef ribs, sprinkle on green onions if using, and serve.
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