Baked Ziti

1 lb. dry ziti pasta
1 onion, chopped
1 lb. lean ground beef
2 (26 oz.) spaghetti sauce
1 ½ cup sour cream
6 slices of provolone cheese
6 oz. mozzarella cheese, shredded
2 Tablespoons grated Parmesan

Bring a large pot of slightly salted water to a boil. Add ziti pasta, and cook until done, about 8 minutes. Drain
In a large skillet, brown onion and ground beef until done. Add spaghetti sauce, and simmer for 15 minutes.
Preheat oven to 350 degrees. Butter a 9x13 inch-baking dish; Layer as follows: ½ of the ziti, provolone cheese, sour cream, ½ sauce mixture, remaining ziti, mozzarella cheese and remaining sauce mixture. Top with grated Parmesan cheese (and a little more mozzarella if desired).
Bake for 30 minutes in the preheated oven, until cheeses are melted.