This is an incredibly tasty soup, despite the fact that the ingredients don't sound that exciting when you first read it. Kale is very nutritious and the soup is low in fat and carbs I believe. It's a really good addition to a diet.
Saute the chopped onion in the olive oil until golden.
Add chopped kale, carrot and shredded brussel sprout and allow to soften a little.
Pour over the stock and bring to the boil, adding the sprig of rosemary. Simmer for 45 minutes adding more stock if required. At this stage the soup will be very watery.
Drain the can of beans, rinse in a colander and then mash with a fork or blend. Add the mashed beans to the soup. This will make th broth a little thicker and give it an interesting texture and flavour.