Season with salt and pepper and dredge the osso buco portions in flour.
Melt a knob of butter and oil in a heavy pan. When hot, brown the osso buco portions for approximately 3-4 minutes, then remove and place in a roasting dish.
Wipe the pan clean and add another knob of butter and a dollop of oil. Then add the onion, carrots, celery and oregano. Sweat for 15-20 minutes.
Add the white wine, reduce, and then add the stock, tomato purée, tinned tomatoes, thyme, and the bay leaf. Season with salt and pepper.
Pour the contents of the pan over the osso buco shanks.
Cover with baking paper and aluminium foil and place in an oven at 130-150°C. Braise for 3 hours until the meat is tender.
Spoon a generous helping of the polenta into a shallow bowl. Place a couple of pieces of the osso buco on top, and spoon over the reduced cooking liquids and vegetables.Garnish with finely chopped parsley and enjoy!