Baked Ziti

	1 lb. dry Ziti pasta	6 ounces provolone cheese, sliced
	1 onion, chopped	1 1/2 cups sour cream
	1 lb. lean ground beef	6 ounces mozzarella cheese, shredded
	2 (26 oz.) jars spaghetti sauce	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 degrees. Butter a 9x13" baking dish. Layer as follows: 1/2 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.