Peel and thinly slice the onion. Peel the carrots and chop into small cubes. Finely slice the celery sticks. Peel and crush one clove of garlic.
Heat a pan for 1 min. When warm, drizzle in 1 tbsp olive oil. Add the chopped veg. Season with salt and pepper. Cook on a low heat for 15 mins, stirring now and then. Crumble in the stock cube and pour in 800 ml boiling water. Drain and rinse the beans. Add to the pan. Pop on a lid and simmer for 5 mins.
Peel the remaining clove of garlic. Put it into a pestle and mortar with a pinch of salt and pound to a smooth paste. Add the oregano leaves. Zest the lemon and juice half of it. Add to the pestle and mortar. Bash together to make an oregano pesto.
Spoon out two ladles of the soup in to a jug or a blender bowl. Blitz using a hand-held blender or blender. Pour it back into the pan. Stir. Taste and season the soup. Ladle it into a couple of bowls and top with the oregano pesto to serve.
[Did you know?]: Butter beans are a good source of protein.