In large skillet, brown chicken and onion then drain.
Return meat to pan; add spaghetti sauce.
Meanwhile, in a medium bowl combine mozzarella cheese and sour cream. Pour hot meat into an ungreased 12 x 8 inch dish.
Spoon cheese mixture over meat. Unroll crescent roll dough over the top of the mixture. Mix parmesan cheese and butter together, then spread over the top of the dough. Bake 18-25 minutes or until golden brown.