In a large bowl, mix all ingredients together. Pour into a greased 9x13 pan. Cover and refrigerate overnight. Bake, uncovered, at 300 degrees for 1 1/2 hours.
Substitutions: Growing up, my brother hated mushrooms. So my mom used Cream of Chicken or Cream of Celery soup instead. Any of these will work. It just depends on what kind of subtle flavoring you're going for.
Also, you can use any kind of shredded cheese you like. We found that, for our taste, cheddar works the best, but feel free to try any others you might prefer.