Baked Ziti

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
6 ounces mozzarella cheese, shredded
2 Tbsp. 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. Butter a 9X13 or 2-8X8 pans. 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.Bake for 30 minutes in the preheated oven, or until cheeses are melted.