It's worth making the olio santo separately ahead of time, just to have on hand for that spontaneous midnight craving. Of course, you can make this pasta without it; just add a bigger pinch of crushed red pepper.
For the hot chili oil (olio santo):
3 medium-hot fresh red chili peppers, such as Fresno; coarsely chopped
3 or 4 dried red chili peppers, such as chile de árbol; crumbled
To make the chili oil, puree all the chilies and oil in a blender until smooth. Pour into a small saucepan and bring to a low boil; simmer 1 minute. Remove pan from the heat to cool and infuse the oil.
Pour the mixture through a mesh strainer into a measuring cup. For clearer oil, avoid pressing down on the solids; if the mix starts to move slowly through the strainer, stir gently with a spoon and/or lift out some of the solids. Line the strainer with a piece of cheesecloth and pour the oil through one more time.
Transfer the oil to an airtight jar or bottle; store in a cool, dark place or in the refrigerator for up to 1 month.
To make the pasta, bring a large pot of water to a boil and add a small handful of salt. Add the spaghetti or pasta of your choice.
Meanwhile, heat the onion, garlic and extra-virgin olive oil in a sauté pan over medium heat until it starts to sizzle; scoop out ½ cup of the boiling pasta water and add to the pan. Cover, lower heat and cook 5 minutes or until the onion and garlic are very tender.
If there’s any water left in the pan, let it bubble away in the uncovered pan until reduced to a tablespoon or so.
Once the pasta is just about done, drain and reserve about ½ cup of the cooking water in a small bowl.
Add the drained pasta to the sauté pan over low heat, along with ½ cup of the cheese; stir enough water to blend with the cheese to coat the pasta.
Serve in bowls drizzled with some parsley, hot chili oil, crushed red pepper and additional cheese to taste.