Wash and trim the leeks making sure to remove any dirt that may be trapped between the layers. Roughly slice the leeks into rounds (about ½ cm thick). Put the washed leeks into a colander and pat dry with kitchen paper or a clean tea towel to remove as much moisture as possible.
Heat the oil in a large heavy-based pan over a low heat. Add the leeks and rosemary. Sweat them very gently until the leeks become translucent and very soft. This will take 10-15 minutes. Stir regularly to prevent the leeks from sticking to the pan and turning brown or charred as this will make for a slightly acrid and bitter taste.
Add the garlic to the pan and continue cooking on a low heat for 3 minutes.
Add the stock, butterbeans, half of the chopped thyme and a good grind of salt and fresh black pepper.
Increase the heat slightly until the soup reaches simmering point.
Now pop the lid on the pan and reduce the heat to allow the soup to cook through. This will take about 15 minutes.
Remove from the heat. Check the seasoning and transfer immediately to serving bowls.
Top with a few thyme leaves and serve.