Cook the rigatoni pasta according to package instructions until just shy of al dente. Drain and set aside.

As the pasta cooks, in a large skillet, heat the olive oil over medium heat. Add the diced guanciale and sauté until it becomes crispy and golden brown, 5-6 minutes. Add the chopped onion and cook until softened (another 2-3 minutes), then add the minced garlic and red pepper flakes. Sauté for an additional minute.

Pour in the crushed tomatoes, season with salt and black pepper to taste, and simmer for 15-20 minutes, allowing the sauce to thicken. Toss the cooked rigatoni into the sauce, ensuring even coating. Sprinkle the grated Pecorino Romano over the pasta and toss again.

Serve hot, garnished with fresh parsley.