Let cheese stand at room temperature for 30 minutes. Meanwhile place noodles in a large bowl. Add enough boiling water to cover by several inches. Let stand for 5 minutes, stirring occasionally. Drain and set aside.
In a microwave-safe 2-quart casserole combine mushrooms, celery, onion, and the 1/4 cup water. Microwave, covered, on 100 percent power (high) for 3 to 4 minutes or until vegetables are tender, stirring once halfway through cooking.
Add cheese and milk to mushroom mixture. Stir until well mixed. Stir in drained noodles and tuna. Microwave, covered, on 100 percent power (high) for 3 to 4 minutes or until heated through, gently stirring once halfway through cooking. Sprinkle with cornflakes.