Preheat your oven to 275 degrees Fahrenheit. Season the ribs with brown sugar, kosher salt, and black pepper. Heat oil in a large, heavy bottom pot over medium heat and brown the short ribs on all sides, about 4-5 minutes. Remove from the pot and set aside.
Add onions, celery, carrot, and garlic to the pot and sauté for about 5 minutes, until the vegetables soften. Now add the tomato paste and stir for an additional 30 seconds. Add red wine, beef broth, and red pepper flakes and stir.
Place the sprigs of fresh rosemary into the pot, then return the browned short ribs to the pot and cover them with a lid. Place pot in the preheated oven. Bake the short ribs, covered, until tender, about 3.5-4 hours.
Remove from the oven and serve.
Notes
Trim off most of the excess fat from the short ribs to avoid a greasy sauce.
If time permits, prepare your short ribs a day ahead and refrigerate in a closed container. The fat from the short ribs will rise to the top. When ready to serve, remove the layer of fat and reheat the ribs and the sauce on the stove-top in a covered pot over low heat.