Cook the pasta in a large pot of boiling water. Add salt and oil. Cook one minute less than package instructions, to ensure the pasta is still stiff enough to stand up.

Drain the pasta and rinse under cold water. Toss pasta with parmesan cheese until evenly distributed, and set aside.

Heat butter in a medium saucepan. Sauté onions and garlic until translucent. Add the sour cream, tomatoes, basil, salt and pepper. Mix and bring to a boil. Set aside.

Preheat oven to 400 degrees Fahrenheit.

Grease a nine-inch springform pan. Take the cooked rigatoni and stand each piece up in the pan until the whole pan is tightly packed with pasta. Pour the mixture and spread evenly over all the noodles.

Bake for 15 minutes. Remove from oven and top with mozzarella cheese. Bake another 15 minutes, until cheese is melted.

Let cool a bit before running a butter knife around the edges of the pan to help remove from the pan.