4 (1 1/2-pound) lamb shanks
1 (19-ounce) can cannellini beans or other white beans, rinsed and drained
1 1/2 cups diced peeled carrot
1 cup chopped onion
3/4 cup chopped celery
2 garlic cloves, thinly sliced
2 teaspoons dried tarragon
1/2 teaspoon salt
1/4 teaspoon freshly ground black pepper
1 (28-ounce) can diced tomatoes, undrained