Bring a large pot of salted water to a boil. Add the bucatini and cook until al dente, according to package instructions.

While the bucatini cooks, heat the olive oil in a large skillet over medium heat.

Add the pancetta or guanciale to the skillet and sauté until crispy, about 5-7 minutes.

Add the onion and garlic to the skillet and sauté until the onion is translucent, about 3-4 minutes.

Stir in the red pepper flakes and cook for 1 minute.

Add the crushed tomatoes to the skillet and bring the sauce to a simmer. Reduce the heat to low and let the sauce simmer for 15-20 minutes, until it has thickened.

Season the sauce with salt and black pepper to taste.

When the bucatini is done, reserve 1 cup of the pasta cooking water, then drain the bucatini and return it to the pot.

Add the Amatriciana sauce to the pot with the bucatini and toss to coat the noodles evenly. If the sauce seems too thick, add a little bit of the reserved pasta cooking water to thin it out.

Serve hot, topped with grated Pecorino Romano cheese and chopped fresh parsley, if desired.