Cook pasta according to package directions until al dente.

In a large skillet, heat olive oil over medium heat.

Add onion and cook until translucent, about 5 minutes.

Add garlic and red pepper flakes, and cook for an additional minute.

Add the can of crushed tomatoes to the skillet, and bring to a simmer.

Reduce heat and let simmer for about 10 minutes.

Season the sauce with salt and pepper to taste.

Drain the pasta and add it to the skillet with the sauce.

Toss to coat.

Serve hot with grated Parmesan cheese, if desired.