Put 1/2 cup 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 2 Tablespoons butter, and when it foams, add the veal shanks to the skillet. Cook until golden, turning once, about 10 minutes. Transfer the shanks to a slow cooker.
Add the 1 cup dry vermouth 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 1 14.5 ounce can diced tomatoes and their juices, 3/4 cup bone broth, 1 red onion, 1 carrot, 1 stalk celery, and 5 sprigs 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. Strain 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 3 Tablespoons parsley, l1 Tablespoon lemon zest, and 1 clove garlic to make a gremolata. Serve the veal shanks topped with the sauce and the gremolata.