Fill a large pot with water and bring to a boil over high heat. Add pasta to boiling water. Cook pasta according to package instructions (approximately 10 minutes.)
In a medium bowl, mix together onion, celery, mushroom soup, and milk
When pasta is cooked, drain pasta and set aside
Spray a large casserole dish with non-stick spray (I like Spectrum coconut oil spray.) Put pasta into dish. Drain tuna and crumble evenly over pasta. Pour mushroom sauce all over, then finish with a layer of cheese
Cover with foil and bake in oven for 25 minutes
After 25 minutes, broil top of casserole for 2 additional minutes to brown the cheese. Serve hot.