Preheat oven to 400 F (200 C). Grease a 13 x 9 inch casserole dish.

In a large pot of boiling salted water, cook the penne for 5 minutes and drain. The pasta will still be slightly hard.

Meanwhile, place tomatoes (juices and all) into a large bowl and crush them using a potato masher.

In a large saucepan heat oil over medium-low heat. Add onions and cook until softened (usually 7-10 min). Stir in garlic. Add crushed tomatoes and stir in pesto. Increase heat to medium, cook for 8-10 minutes or until sauce is hot.

In a large bowl toss the sauce with the pasta and scrape into prepared baking dish. Top with shredded mozzarella and cheddar. Bake uncovered for 15-20 minutes, or until cheese is melted and browned.