Pat the shanks dry and season well. Dust them all over with the flour, shaking off the excess.
Heat the oil in a large frying pan over a high heat. Brown the shanks hard on all sides, 8–10 minutes in total, working in two batches so the pan stays hot. Transfer to the slow cooker.
Turn the heat down to medium and cook the onion, carrot and celery in the same pan for 5 minutes, then add the garlic for a further minute. Tip into the slow cooker.
Pour the wine into the hot pan and let it bubble for a minute, scraping up everything stuck to the base. Pour this over the shanks.
Add the stock, tomato paste, rosemary, bay leaves and brown sugar to the cooker. The liquid should come about two-thirds of the way up the shanks.
Cover and cook on low for 8 hours, or high for 5. The meat is ready when it pulls away from the bone with no resistance.
**Lift the shanks out carefully** and keep them warm. Strain the liquid into a saucepan, discarding the rosemary stalks and bay leaves.
Boil the liquid hard for 8–10 minutes until reduced by about half and thick enough to coat the back of a spoon. Taste and season.
Return the shanks to the sauce to warm through, and serve over mashed potato or creamy polenta with the sauce spooned over.