Trim the roots and top 3cm off the top of the leeks. Halve lengthways and rinse well under a cold tap. Chop into 1cm-thick pieces. Peel the onion and thinly slice it. Peel the garlic cloves and crush or finely slice them. Grate the lemon zest.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the leeks, onion, garlic and lemon zest. Sprinkle in a pinch of salt and pepper. Fry for 10 mins, stirring occasionally, till the vegetables have started to soften.
Drain the cannellini beans and rinse them. Tip into the pan with the leeks and crumble in the stock cube. Pour in 750ml boiling water and stir well. Bring to the boil, then reduce to a simmer and cook for 10 mins.
Use a stick blender to partly whizz the soup and thicken a little. No stick blender? Simply use a masher to break up the beans a little.
Pick the mint leaves and finely shred them. Stir into the soup along with the juice of half the lemon. Have a taste and add a little more salt and pepper if needed.
Ladle the soup into bowls and drizzle over 1 tbsp olive oil. Finish with a crack of pepper and serve.