In a large sauce pot over medium heat, warm the olive oil. Add the onion and pancetta and cook slowly, until onions are soft and translucent and the pancetta fat is rendered. Add the tomatoes, hot red pepper flakes and season with salt and pepper.
Boil a large pot of water; salt generously. Cook the bucatini until it is soft but still firm.
Divide the pasta between 4 warmed pasta bowls. Ladle some sauce on top, and serve with grated Pecorino Romano.