Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Sautee mushrooms and set aside. In the same hot pan,make a behcamel-type sauce starting with a roux (2 tb flour, 2tb butter), adding a can of evporated milk, salt and white pepper. Thicken and stir in mushrooms.
Add cheese and melt, stirring. When cheese is almost melted, stir in tomatoes with green chile peppers and green onion. Simmer over low heat for 2 to 3 minutes, stirring.
Add spaghetti and mix well, then stir in chicken. Heat through and serve.