Preheat oven to 350°F. Place flour in a shallow plate. Salt and pepper both sides of the veal shanks and dredge in the flour, shaking off excess flour.
Heat olive oil on medium high in a large dutch oven. When oil is hot, brown the veal shanks on both sides, about 3 minutes per side. Remove from the dutch oven and set aside on a plate.
Add carrots, celery, onions, garlic, bay leaves and thyme to the pot, stir well scraping up the bits of meat and flour at the bottom.
Stir in the tomato paste and cook for 1 minute. Add the red wine and cook, allowing the wine to evaporate, about 5 minutes. Pour in the diced tomatoes, chicken stock and red pepper flakes and bring to a boil.
Nestle the veal shanks in the sauce. Cover with a lid and place in the oven for 2 1/2 hours. Remove the pot from the oven and place on the stove top. Remove the veal shanks from the pot and simmer the sauce on the stove top for about 15 minutes until thickened.
Meanwhile to prepare the gremolata, finely chop the parsley and combine with the garlic and lemon zest. Mix well.
To serve, spoon some polenta on each plate, top with osso buco and ladle some sauce on top. Sprinkle with gremolata.
* To make polenta, bring the water and milk to a gentle boil, whisk in cornmeal flour and salt, stir until combined. Lower heat, cover and cook for about half an hour, stirring frequently. When polenta is ready, stir in parmesan cheese and serve.