In a large skillet, cook chicken in butter 3-4 minutes on each side or until meat juices run clear. Place chicken in a greased 3-cup baking dish. Meanwhile, in a large bowl, combine the cream cheese, mushrooms, onion, salt and pepper. Spoon over chicken.
Unroll dough into a single long rectangle; seal seams and perforations. If necessary, trim dough to fit top of dish and patch together by overlapping edges. Pinch edges to seal. Place over filling. Bake, uncovered, at 350° for 20 minutes or until heated through.