In a large saucepan, bring water and bouillon to a boil, stirring to dissolve. Add the next eight ingredients; return to a boil. Reduce heat; cover and simmer for 5 minutes or until vegetables are tender. Dissolve corn starch in milk; add to the vegetable mixture, stirring constantly. Bring to a boil; boil 2 minutes or until thickened. Remove from the heat; stir in Swiss cheese and mayonnaise until cheese is melted. Fold in noodles and flaked tuna. Pour into a greased 2-1/2 quart baking dish. Brown bread crumbs in butter. Sprinkle on top of casserole. Bake, uncovered, at 350 for 25-30 minutes or until heated through.