To a large frying pan, add the oil, onion and garlic. Season with salt and pepper and fry over a medium/low heat.
Meanwhile, add the spaghetti into heavily salted boiling water. Set your timer to the packet time minus 2 minutes.
Once the onion and garlic have softened, after around 6 minutes, add the spinach, dried parsley and 200ml of salty pasta water.
After around 2 minutes, add the contents of the frying pan into a blender and blend until smooth.
Pour the spinach sauce back into the pan, reducing heat to low. Transfer the al dente spaghetti into the pan and add two thirds of the parmesan. Stir until well combined, adding a few ladles of pasta water to loosen.
Serve and top with the remaining parmesan, an extra crack of black pepper or optional chilli flakes. Dive in!