1) Add the pancetta to a heavy large frying pan over medium heat. Add olive oil and saute until golden brown, about eight minutes.
2) Add the onion and saute until tender, about five minutes. Season with salt. Add the garlic and red chilli flakes. Saute until fragrant, about 30 seconds. Stir in the tomato puree. Simmer uncovered over medium-low heat until the sauce thickens slightly and the flavors blend, about 15 minutes. Season the sauce with salt, to taste.
3) Meanwhile, boil the linguine in a large pot of boiling salted water until tender but still firm to the bite, stirring occasionally, about eight minutes. Drain, reserving 250ml of the cooking liquid.
4) Toss the linguine with the sauce in the pan, adding some of the reserved cooking liquid until the pasta is moist but being careful not to dilute the sauce. Toss with the cheese. Season with salt, to taste, and serve.