Slice the onions, garlic and leeks, reserving the darker green tops (or the top 3 inches) for later. Set a heavy-based casserole pan on a medium heat. Sweat the onion, leeks and half the sliced garlic in half the olive oil with a big pinch of salt for 5-8 minutes until translucent and soft.

Tip in the drained butter beans, vegetable stock and mustard. Bring to a gentle simmer and cook for 10 minutes.

To make the frazzled leek tops, chop the parsley and zest the lemon. Slice the leek tops from earlier as finely as possible. Set a small frying pan on a medium-high heat and add the remaining olive oil. Fry the sliced leek tops in the oil until caramelised and slightly frazzled at the edges, about 5 minutes, keeping an eye on them and stirring frequently. Turn off the heat and stir in the remaining chopped garlic, the lemon zest and parsley and let them gently sizzle in the residual heat of the oil. Season with salt and add an extra drizzle of oil if you'd like.

Use a stick blender to puree the soup, squeeze in the lemon juice and taste for seasoning. Dish the soup up into bowls and top with a spoonful of the frazzled leek tops. Serve with crusty bread, if using.