Baked Ziti

I found this on allrecipes.com...and what a tasty recipe!

INGREDIENTS:
* 1 pound dry ziti pasta
* 1 onion, chopped
* 1 pound lean ground beef
* 2 (26 ounce) jars spaghetti sauce
* 6 ounces provolone cheese, sliced
* 1 1/2 cups sour cream (used reduced-fat)
* 6 ounces mozzarella cheese, shredded (used whole 8oz bag)
* 2 tablespoons grated Parmesan cheese

DIRECTIONS:
Preheat oven to 350 degrees.

1. Bring a large pot of lightly salted water to a boil. Add ziti pasta, and cook until al dente, about 8 minutes; drain.
2. In a large skillet, brown onion and ground beef over medium heat. Add spaghetti sauce, and simmer 15 minutes.
3. Spray 13x9 baking dish with Pam. 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.
4. Bake for 30 minutes in the preheated oven, or until cheeses are melted.