Preheat the oven to 375°F (190°C).
Season the veal shanks generously with salt and pepper, and then dredge them in flour, shaking off any excess.
Heat the olive oil in a large Dutch oven over medium-high heat. Add the veal shanks and sear until browned on both sides, about 5 minutes per side. Remove the shanks from the pot and set them aside.
Add the onion, celery, and carrots to the pot and cook until the vegetables are soft and translucent, about 5 minutes. Add the garlic and cook for another minute.
Pour in the canned tomatoes and stock, and then stir to combine. Add the bay leaves, thyme, and lemon zest.
Return the veal shanks to the pot, making sure they are submerged in the sauce. Cover the pot and transfer it to the oven.
Bake for 2 to 3 hours, or until the meat is tender and falling off the bone.
Remove the pot from the oven and let it sit for a few minutes. Remove the bay leaves and thyme sprigs.
Serve the Osso Buco hot, garnished with fresh parsley or lemon zest, and with a side of creamy polenta or crusty bread.