1. Bring a large pot of lightly salted water to a boil.
2. Add pasta and cook until al dente, about 8 minutes; drain.
3. In a skillet, brown onion and beef over medium heat; add spaghetti sauce and simmer 15 minutes.
4. Preheat oven to 350 degrees F.
5. Butter a 9x13 baking dish and layer as follows: 1/2 of the ziti, provolone cheese, sour cream, 1/2 sauce mixture, remaining ziti, mozzarella cheese and remaining sauce mixture.
6. Top with grated Parmesan cheese.
7. Bake for 30 minutes or until cheese is melted and lightly browned.