Pre heat the oven to 200 degrees Celsius and line a tray with baking paper.
Heat the butter and olive oil over a medium heat and cook the leeks, onion and garlic for around 10 minutes or until nice and soft. Don’t brown them.
Add the rest of the ingredients to the pan with a generous pinch of salt and pepper, bring to the boil then lower to a simmer and cook covered for 30 minutes.
Whilst the soup cooks coat the cubed potatoes in a drizzle of olive oil and some salt and pepper then place onto the prepared tray and roast until golden brown.
Once the potatoes are soft, add the lemon juice then use a stick blender to blend the soup until smooth, taste for seasoning then serve with the crispy potatoes, a dollop of sour cream and some fresh chives.