Baked Ziti

1 Pound dry ziti pasta
1 Onion, chopped
1 Pound lean ground beef
2 (26 oz) Jars spaghetti sauce
6 Ounces provolone cheese, sliced
1 1/2 Cups sour cream
6 Ounces mozzarella cheese, shredded
2 Tablespoons grated Parmesan cheese

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 15 minutes.
Preheat the oven to 350�F.
Butter a 9" x 13" 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.
Place dish in oven and bake for 30 minutes in the preheated oven or until cheeses are melted.