Cook pierogi according to package directions; drain. Meanwhile, toss chicken with salt and pepper. In a large nonstick skillet, heat 1 tablespoon butter over medium-high heat; saute chicken and onion until chicken is no longer pink. Remove from pan., In same pan, heat remaining butter over medium heat; saute pierogi until lightly browned. Stir in chicken mixture; sprinkle with cheese. Cover; remove from heat and let stand until cheese is melted.