Braised Lamb Shanks

Preheat oven to 350°. Season lamb shanks with salt and pepper. In a large Dutch oven over medium-high heat, heat oil. Add lamb and cook until golden all over, about 4 minutes per side. Work in batches as necessary. Remove lamb from pot and place on a plate.  In the pot, melt 2 tablespoons butter over medium heat. Add onion and carrots and cook until soft, 4 minutes. Add garlic and cook until fragrant, 1 minute more, then add tomato paste and stir until vegetables are coated.  Pour in broth and tomato sauce, then return lamb shanks to pot. Add rosemary and bring to a boil. Cover and transfer to oven. Cook until lamb shanks are completely tender, 2 hours. Remove lamb from pot and place on a cutting board. Tent with foil to keep warm. Discard rosemary.  Return pot to stove over medium heat. Add wine and bring to a boil. Reduce heat and let simmer until thickened, 10 minutes. Stir in remaining 2 tablespoons butter.  Serve lamb with mashed potatoes and spoon sauce on top.