Place the basil, garlic, pine nuts and olive oil together with some salt in a mini-chopper and whizz, then push any unblended basil leaves down with a spatula and whizz again until you have a smooth purée.
Then transfer the purée to a bowl and stir in the grated Pecorino Romano cheese.
If you don't have a mini-chopper, you can use a large pestle and mortar to pound the basil, garlic and pine nuts to a paste.
Slowly add the salt and cheese, then very gradually add the oil until you have obtained a smooth purée.
If you want to make pesto in advance - cover it with cling film, making sure it actually touches the surface of the sauce.
You can keep it in the fridge for a week.