Preheat the oven to 325 degrees F.
Heat the oil in a large Dutch oven or wide pot over medium-high heat.
Season the shanks with salt and pepper. Now dredge the shanks in the flour, Shake any excess off.
Add the shanks to the hot pan, and cook on all sides until browned. Cook in batches if needed to avoid steaming the meat.
Remove the shanks and set aside.
Lower the heat to medium. Add the onions, carrots, celery, garlic, fennel, thyme, rosemary, bay leaves and one teaspoon of salt to the pot. Cook until softened, while stirring often, about 4-5 minutes.
Add the tomato paste and cook for 2 minutes. Add the wine, scraping with a spoon to deglaze the pan.
Return the shanks to the pot. Add enough chicken stock to cover the shanks around ¾ of the way.
Taste the cooking liquid and adjust with salt and pepper if needed.
Cover and braise in the oven until the meat is fork tender. Around 4 to 5 hours.
Remove from the oven once for tender, cover and set aside while you make the polenta and gremolata.