Preheat oven to 300°.
To prepare lamb, sprinkle lamb evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Heat a large, wide Dutch oven over medium-high heat. Add lamb, and cook for 12 minutes, browning on all sides. Remove lamb from pan.
Add onion, carrot, and celery to pan; sauté 8 minutes or until lightly browned. Add garlic, and sauté 1 minute. Add red wine; bring to a boil. Cook 2 minutes or until most of liquid evaporates. Return lamb to pan; stir in tomatoes, beef broth, and 1 tablespoon of chopped rosemary. Bring to a boil.
Cover Dutch oven; place in oven. Bake at 300° for 2 hours or until lamb is tender. Remove lamb from pan; set aside and keep warm. Add remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper to pan; bring to a boil over high heat. Cook until sauce is reduced to about 3 1/2 cups (about 30 minutes), stirring frequently. Combine water and cornstarch in a small bowl, stirring with a whisk. Add cornstarch mixture to pan; cook 30 seconds or until sauce thickens, stirring constantly.
To prepare polenta, bring chicken broth, 1 teaspoon rosemary, and 1/4 teaspoon pepper to a boil in a large saucepan. Gradually add cornmeal, stirring constantly with a whisk. Reduce heat to medium; cook 4 minutes or until thick, stirring constantly. Remove from heat, and stir in cheese. Serve immediately with lamb and sauce.