Preheat oven to 325°. Season short ribs generously with salt and pepper. Heat olive oil in a large Dutch oven over medium high heat. Add ribs and brown on all sides. Remove the ribs and reserve. Add the celery, carrot, onion, mushrooms and garlic to the pot and saute until tender. Add tomatoes, broth, wine, tomato paste, balsamic vinegar, rosemary and bay leaf. Return the ribs and any accumulated juices to the pot. Heat to boiling. Cover and transfer to the oven. Roast about 2-3 hours until ribs are very tender. Check after 2 hours to see if additional broth needs to be added. To thicken sauce, remove the cooked ribs, place the pot on the stove over medium high heat. Bring liquid to a boil, whisk in the flour slurry and cook and stir until the mixture thickens. Serve with ribs over polenta.