French green beans, so much more vibrant in flavour than the muddy Kenyan ones that dominate the supermarket shelves in Britain, are coming to the end of their season. We have been eating them whenever we can.
Some people assert that you should never steam green vegetables, or put the lid on the pan of boiling water. The science behind that theory appears to be that the atmosphere inside a closed pan has higher acidity -- and acidity causes the vegetables to lose greenness, turning an unappetising khaki. I am not sure about this effect from my own experience; but perhaps the water in North London is low in acidity. I usually steam these vegetables (unless there is an opportunity to cook them in boiling water I am about to use for pasta), because I have read that steaming retains more nutrients.
However, I boil vegetables here in France, where we do not own a steamer. I give green beans three and a half minutes from the moment I plunge them into the boiling water.
Whether or not to salt the water is another difficult question. Harold McGee says that salt speeds softening, and therefore reduces the time that the vegetables have to spend in the colour-sapping pan. But I usually leave it out.