Lamb & Sweet Potato Bake

2 pounds lean lamb shoulder, cut in 1-inch cubes
1 can (13 3/4 ounces) chicken broth, or 1 3/4 cups
1 can (4 ounces) sliced mushrooms
2 teaspoons salt
1/8 teaspoon pepper
3 medium sweet potatoes, peeled and cut in 1-inch cubes
1 pound fresh trimmed and cut green beans
1 T plus 1 teaspoon cornstarch
2 T water

 Brown lamb in a large heavy skillet; place in a greased 2 1/2-quart baking dish. Drain off fat in skillet; stir in chicken broth, mushrooms with liquid, salt, and pepper. Bring to a boil; pour over lamb in baking dish. Stir in sweet potato cubes and green beans. Cover and let stand for 30 minutes. Bake at 350° for 1 1/2 hours, or until meat and vegetables are tender. Stir together cornstarch and water; stir into liquid in casserole. Bake, uncovered, stirring once, for 8 to 10 minutes longer, or until gravy is thickened.