Let's start by bringing a large saucepan of water to boil. For the best texture, use 1 litre of water per 100g of pasta, and add in a large handful of good quality rock salt.
Once the water's boiling, add your pasta and cook for a few minutes LESS than the packet directions indicate (the pasta will continue to cook when it's added into the sauce). Drain the pasta but 4 cups of the pasta water to use in the sauce. Set aside.
Meanwhile, heat the olive oil in a large pan over a high heat. Add the onion, chillis and guanciale/pancetta or bacon pieces, and stir fry until gorgeously fragrant.
The next step is to add the passata and 4 scoops of pasta water. Stir everything through then reduce heat to low and simmer for a few minutes.
Now for the fun part! Place the drained pasta on top of the sauce, but DON'T mix through yet. First, top the pasta with your grated pecorino romano cheese and gently mix it through the pasta (avoiding the sauce as much as possible).
Once the pasta is coated with that wonderfully melted cheese, you can start to slowly mix it in with the sauce. It should bind nicely to the cheese-coated pasta.
If you decide the sauce is too dry, you can add more pasta water and mix it through until the pasta is nicely coated.
To serve, portion out the pasta into serving bowls, leaving some of the cooked pancetta to place on the top as a garnish.