Cook spaghetti in hot salty water and then drain it but save some starchy water.
While spaghetti is cooking, just simply blend other ingredients (parsley, garlic, onion, cherry tomatoes, olive oil, salt and pepper) to smooth pesto. (adjust the quantities to taste)
Add parsley pesto to cooked spaghetti and stir and add little starchy water to make it moist. Leave covered up for a minute so that pesto ingredients cook from the heat of hot spaghetti and start to smell good. Then stir in grated Grana Padano cheese.
Serve straight away, topped with some additional grated cheese and drizzle with olive oil.