Heat the oil in a large frying pan and add the pancetta, cooking on a high heat until just crisp, stirring in the chilli flakes for the last minute of cooking.
Remove the pancetta from the pan and add the onions to the oil that has been left in the pan.
Cook the onions on a low heat for 8-10 minutes, until really soft. Add in the tinned tomatoes, bring to the boil, then simmer for 10 minutes, stirring occasionally until thickened.
Cook your pasta at this point too. I cook my bucatini in boiling water for 7 minutes, so that it still retains a bite of bite, then drain, reserving half a cup of the pasta water.
Once the pasta is cooked and the tomatoes are thickened, add the pasta to the pan with the tomatoes. Add three-quarters of the percorino, the cooked pancetta and the black pepper, then toss everything together to coat the pasta. Add in some of pasta water if the sauce is a little too dry for you (although the sauce should stick to the pasta, without pooling at the bottom of the pan).
Serve topped with the reserved perorino and parsley.