Baked Ziti

1 pound dry ziti pasta
1 onion, chopped
1 pound lean ground beef (seasoned)
2 (26 ounce) jars spaghetti sauce
8 slices provolone cheese
1 1/2 cups sour cream
6 ounces mozzarella cheese, shredded
2 tablespoons grated Parmesan cheese

1. Bring a large pot of lightly salted water to a boil. Add ziti pasta and cook until al dente, about 7-8 minutes; drain.
2. In a large skillet, brown onion and ground beef over medium heat. Add spaghetti sauce, and simmer 15 minutes.
3. Preheat oven to 350. Spray 2 8x8 pans with non-stick cooking spray. Layer as follows in each pan: 1/4 of the ziti, 1/4 of the sauce, 4 slices of provolone cheese, 1/2 of sour cream, 1/4 of ziti, 1/4 of mozzarella cheese, and 1/4 of sauce mixture. Top with grated Parmesan cheese and remaining mozzarella cheese.
4. Bake covered for 30 minutes in the preheated oven, or until cheese is melted.