Bring a large pot of lightly salted water to a boil, and cook ziti pasta 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 9x13 inch baking dish and layer with 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 cheeses are melted.