Heat water with salt and oil in a medium/large pot, but don’t get to a boil.
Add the flour “a fontana” (=sprinkling it over the water little at the time) and whisk vigorously. Whisk until well mixed.
Cover with the dish cloth (in contact with the paste), leaving a chink. Place the pot over the smallest of your stoves, at the minimum heat. Let cook 30-40 minutes.
Meanwhile grate the pecorino cheese and julienne guanciale. Brown the guanciale in a small pan (no oil, it will release its own) until crunchy.