Stir together all casserole ingredients, pour into greased 12x8x2-inch baking dish. Bake at 350 degrees for 30-35 minutes or until knife inserted in center comes out clean. To make the sauce, combine cornstarch with 1/4 cup broth. Heat remaining broth to boiling in a saucepan; gradually whisk in cornstarch, mushrooms and green onions. To serve cut casserole into squares and top with mushroom sauce.