Heat the oil in a large saucepan and add the onion, leeks, potatoes and butter. Cook for 5-10 minutes. Add the stock and bring to the boil. Cover and simmer for 20 minutes, until the potato is soft. Meanwhile, grill the bacon until crispy, then chop into strips.
Once the soup is ready, remove some of the potato chunks, cool slightly and chop into small cubes. Set aside.
Blend the soup in a food processor until almost smooth, then divide between bowls. Top with the potato chunks, bacon strips and parsley. Season with freshly ground black pepper and serve drizzled with the olive oil.
Cook's tip: for vegetarians, serve the soup with crispy onions inplace of the bacon.