Put the borlotti beans in a large bowl, cover with cold water and let the beans soak overnight or for at least 6 hours.
Drain the beans and tip them into a large saucepan. Using the flat side of a large knife, press down on the whole garlic cloves so that their papery skins tear and begin to come away. Peel them and chuck the bruised cloves on top of the beans. Now take a popsock and in it pop, appropriately enough, the sprigs of rosemary and cut-up onion. This will stop the needles (which turn bitter on boiling) from infiltrating the soup (very irritating between the teeth, too) but allow their resiny fragrance to seep through. I also find it better not to have slimy onion skins all over the place later. Cover everything generously with cold water, clamp on a lid and bring to the boil. Once it’s started boiling, turn the heat down and simmer for an hour. Check the beans to see how cooked they are and, only when they’re tender, add salt to taste.
Chuck out the corpsed popsock and its contents. Remove a mugful of beans – or more if you want a very thick soup – and tip into a blender (my preference) or processor, along with a tablespoonful of tomato concentrate and 250ml / 1 cup of the beancooking liquid and liquidise.
Now, add the 3 tablespoons of oil to a small saucepan and grate (I always use my fine Microplane for this) or squeeze in the sixth clove of garlic. Cook over a low to medium heat until soft but not coloured and then stir in the finely chopped rosemary. Cook for another scant minute, add the liquidised soup and cook for a minute or so, then tip into the large pan of beans. Bring back to the boil and add the ditalini, cooking them according to packet instructions. Drizzle with extra-virgin olive oil and serve.
