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 sausage over medium heat. Set aside. Sauté the remaining vegetables until lightly browned. Combine the pasta, spaghetti sauce, vegetables and sausage in an extra large pot or bowl.
Preheat the oven to 350 degrees F (175 degrees C). Butter a 9″x13″ baking dish.
Layer as follows: Ziti and sauce mixture, all sour cream, provolone and mozzarella cheese, ziti and sauce mixture, provolone and mozzarella cheese. Garnish with grated Parmesan cheese.
Bake for 30 minutes in the preheated oven, or until bubbly and cheese is melted.