Pat the shanks dry with a paper towel and lightly dredge in flour, shaking off the excess. Heat the oil and butter in a Dutch oven or medium pot and brown the shanks on both sides and transfer to a plate. Set aside.
If making the gremolata, combine the ingredients in a small bowl, cover and refrigerate until ready to use.
Add the pancetta or bacon and cook until browned, then add the onions and cook for 5-7 minutes until soft and translucent. Add the carrots, celery, and garlic and cook another 3-4 minutes until softened. Add the wine and boil until reduced by half. Add the broth, seasonings, and tomato paste.
Return the browned shanks to the pot, bring to a boil, reduce the heat to a very low simmer, cover and simmer for 1 1/2 to 2 hours or until the meat is fork tender. Add salt and pepper to taste. Remove and discard the bay leaf.
Serve with creamy polenta or saffron risotto and, if using, sprinkle with gremolata. (See blog post for other serving recommendations.)