Preheat the oven to 350 F. In a heavy bottomed oven proof pan or dutch oven, heat the oil and butter over medium heat. Season the lamb shanks liberally with salt and black pepper. Cook until browned on all sides, turning frequently. Transfer the lamb shanks onto a plate and set aside while preparing the vegetables and sauce.

In the same pan add the onions, carrots, celery and garlic and sauté over medium heat until softened, stirring often so that it does not burn. Adjust heat if necessary.

Add the wine, tomatoes, tomato paste, beef broth, rosemary, thyme and lemon zest. Bring to a boil and add the reserved lamb shanks. Cover with a lid, transfer to the oven and braise for 2 hours. Remove the lid after 1½ hours and continue to roast the additional 30 minutes until the lamb is very tender and the sauce has reduced and thickened up a little.

Divide the lamb shanks into four shallow dishes with some of the sauce spooned over the top and along side of the lamb.