Bring large pot of lightly salted water to a boil. Add ziti pasta and cook until al dente, about 8 minutes; drain. In a large skillet, brown onion and ground beef over medium heat. Add spaghetti sauce, and simmer 15 minutes.
Preheat the oven to 350 degrees F. Butter a 9×13 baking dish. Layer as follows: 1/2 of the ziti, provolone cheese, sour cream, 1/2 sauce mixture, remaining ziti, mozzarella cheese, and remaining sauce mixture. Top with Parmesan cheese. Bake for 30 minutes or until all cheeses are melted.