Take a large saucepan and melt the butter before adding the onion, cooking until soft and transparent.

Add the diced potato and cook for another ten minutes.

Add the stock and the milk and bring to a gentle simmer, cooking until the potatoes are soft.

Add the chopped lovage leaves and remove from the heat.

Using a hand blender, puree the soup. Add more stock or water if you feel the soup is too thick.

Place the saucepan back on the heat and season to taste.