Bring 3 litres of water to the boil with the garlic cloves, celery, porcini and parsley, plus a pinch of salt. Allow to simmer until the garlic is very soft, about 10–15 minutes.
Add the pasta and saffron.
Simmer and keep stirring so the pasta cooks evenly. It should take a minute or two longer than it would normally. The idea is for almost all of the liquid to evaporate evenly as the pasta cooks. It is better to keep the pan on the dry side and turn the pasta over in order to cook uniformly than have it too wet. Add a ladleful of water from time to time if needed.
When thick and cooked add a drizzle of oil, or indeed a knob of butter if you’d rather, and the cheese and stir. Allow to sit for 3 minutes with a lid on before serving so that the sauce finishes thickening and soaks even more into the penne.