Put the olive oil, garlic and chilli flakes (if used) in a pan at a low temperature. The goal is to flavor the olive oil and you have achieved this when the garlic starts to turn golden in some spots.
At the same time, boil the spaghetti for 2 minutes less than recommended on the package (if, for example, it says 9, boil them for 7, we want them to be al dente).
Once the garlic is golden in some spots, add the spaghetti and half a cup of the water in which they boiled and stir.
Add salt, nutritional yeast and parsley and stir.
Serve immediately, plain or adding nutritional yeast, freshly ground pepper and parsley.
Notes
If you want to make it spicier, add some spice chilli flakes.
The best result is obtained if you heat the olive oil with the garlic and immediately start boiling the spaghetti. So, when the garlic has started to turn golden, the spaghetti will be ready for the pan.