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. Preheat oven to 350 degrees F (175 degrees C.)
Melt butter in a large, deep skillet over medium high heat. Saute the chicken, onion and mushrooms until chicken is no longer pink, and juices run clear. In a large bowl, combine cooked pasta, peas, milk, condensed soup and 1 cup of the cheese. Stir in the chicken mixture. Pour into a 3 quart baking dish and sprinkle with remaining 1 cup of shredded cheese.
Bake in preheated oven for 20 to 25 minutes, or until sauce is bubbly.
