Put the flour in a wide, shallow dish. Season the veal shanks all over with salt and pepper and dredge in the flour; shake off the excess flour.

Heat a 12-inch skillet over medium heat. Add the butter, and when it foams, add the shanks to the skillet. Cook until golden, turning once, about 10 minutes. Transfer the shanks to a slow cooker.

Add the wine to the skillet. Scrape up any browned bits from the bottom of the skillet and pour the contents of the skillet into the slow cooker. Add the tomatoes and their juices, chicken broth, onion, carrot, celery, and thyme. Cover and cook on low heat for 6 to 8 hours—the meat will be very tender and almost falling off the bone.

Transfer the shanks to a platter and cover with foil to keep warm. Pour the sauce from the slow cooker into a large skillet. Simmer over medium heat until reduced to about 2 cups, 10 to 15 minutes. Season to taste with salt and pepper.

Meanwhile, in a small bowl, combine the parsley, lemon zest, and garlic to make a gremolata. Serve the veal shanks topped with the sauce and the gremolata.