Baked Ziti

My MIL Susan made this and it is delicious!

1# dry ziti pasta
1 onion, chopped
1# lean ground beef
2 (26 oz) jars spaghetti sauce
6 oz. provolone cheese, sliced
6 oz. mozzarella cheese, shredded
1 1/2 c. sour cream
2 Tbsp. grated parmesan cheese

Bring 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, saute the onion and ground beef over medium heat. Add spaghetti sauce & simmer 15 minutes.
Preheat oven to 350. Butter a 9"x13" baking dish (Susan uses a dish with very high sides). Layer as follows: 1/2 of ziti, provolone, sour cream, 1/2 sauce mixture, remaining ziti, mozzarella, and remaining sauce mixture. Top with grated parmesan cheese.
Bake for 30 minutes, covered, in the preheated oven, or until cheeses are melted.