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. Season with salt and pepper. Drain.
Stir in marinara 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 crescent dough, press seams together and lay over cheese mixture. 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.