Tuna Mushroom Casserole

In a large saucepan, bring water and bouillon to a boil; stir until bouillon is dissolved. Add the next eight ingredients; bring to a boil. Reduce heat; cover and simmer until vegetables are tender, about 5 minutes. , In a small bowl, combine cornstarch and milk until smooth; gradually add to vegetable mixture. Bring to a boil; cook and stir until thickened, about 2 minutes. Remove from the heat; stir in cheese and mayonnaise until cheese is melted. Fold in noodles and tuna. , Pour into a greased 2-1/2-qt. baking dish. In a small skillet, brown bread crumbs in butter; sprinkle over casserole. Bake, uncovered, at 350° until heated through, 25-30 minutes.