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.
Melt butter in a large frypan over medium heat. Cook chicken until no longer pink and juices run clear. Transfer chicken to a cutting board and thinly slice; set aside. Saute onion until tender and translucent. Add mushrooms and gradually stir in cream. Cook, stirring constantly, for 7 to 10 minutes, or until sauce is thickened. Stir in chicken and season with salt and pepper. Cook 5 minutes, or until heated through. Toss with cooked pasta until evenly coated.