Preheat the oven to 325°F. Season the veal shanks with salt and pepper, then dust with flour, shaking off any excess.

In a large Dutch oven, heat the olive oil and butter over medium-high heat. Add the veal shanks and brown on all sides, about 3-4 minutes per side. Remove the shanks and set aside.

In the same pot, add the onion, carrot, celery, and garlic. Sauté until the vegetables are softened, about 5-7 minutes.

Pour in the white wine, scraping up any browned bits from the bottom of the pot. Let it simmer for about 2-3 minutes.

Return the veal shanks to the pot. Add the beef broth, diced tomatoes, thyme, and bay leaf. Bring to a simmer.

Cover the pot and transfer it to the preheated oven. Cook for 2 to 2.5 hours, or until the meat is tender and falling off the bone.

While the osso buco is cooking, prepare the gremolata by mixing the lemon zest, parsley, and minced garlic in a small bowl.

Once done, remove the pot from the oven and discard the bay leaf. Serve the osso buco hot, topped with gremolata.