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 medium heat.
Add spaghetti sauce, and simmer for 15 minutes.
Heat the oven to 350 degrees F. Butter a 9 x 13 x 2-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.