Bring a 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 mediun heat. Add spaghetti sauce, and simmer 15 minutes.

Preheat the oven to 350°F. Butter a 9×13 inch 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 grated Parmesan cheese.

Bake for 30 minutes in the preheated oven, or until cheese are melted.