Bring pot of lightly salted water to boil. Add ziti pasta and cook about 8 min. Drain.
Brown onion and beef in skillet over medium heat. Add spaghetti sauce and simmer 15 min.
Preheat oven to 350 degrees. Butter a 13x9" baking dish. Layer as follows: half of ziti, provolone, sour cream, half of sauce mixture, remaining ziti, mozzarella, and remaining sauce. Top with Parmesan. Bake covered for 30 min til cheeses are melted.