Heat oven to 375°F. In large skillet, cook ground beef and onion over medium heat for 8 to 10 minutes or until beef is thoroughly cooked, stirring frequently. Drain. Stir in pasta sauce; cook until thoroughly heated.
Meanwhile, in medium bowl, combine mozzarella cheese and sour cream; mix well.
Pour hot beef mixture into ungreased 9 1/2 or 10-inch glass deep-dish pie pan or 11x7-inch (2-quart) glass baking dish. Spoon cheese mixture over beef mixture.
Unroll dough over cheese mixture. (If using pie pan, separate dough into 8 triangles; arrange points toward center over cheese mixture, crimping outside edges if necessary.) In small bowl, mix Parmesan cheese and butter. Spread evenly over dough.
Bake at 375°F. for 18 to 25 minutes or until deep golden brown.