In a saucepan with a lid, bring roughly 2 litres (3.½ pints) of water to the boil with a dessertspoon of salt.
Put the green beans into the water and cover until the water comes back to the boil, then remove the lid and cook until al dente.
Drain the beans and return to the saucepan with the butter. Season with pepper and stir in the Parmesan. Keep over the heat until the Parmes