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 skillet 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.