Bring large pot of lightly salted water to a boil. Add Ziti pasta and cook until al dente, about 8 minutes. Drain well
In a large skillet, ground beef and onion over medium heat. Add spaghetti sauce and simmer for 15 minutes.
Preheat oven to 350. Butter a 9x13 inch baking dish and layer as follows: 1/2 ziti, 1/2 sauce and meat mixture, provolone cheese, sour cream, remaining ziti, remaining sauce and meat mixture, mozzarella cheese then top with grated parmesan cheese.
Bake uncovered for 30 minutes or until cheeses are thoroughly melted.