Peel the potatoes and onion and cut them into rough 1inch (2.5cm) pieces. Split the leeks in half lengthwise and rinse out any soil, and cut into 1inch (2.5cm) pieces.
In your soup pot, dry-fry the bacon lardons over a medium-high heat for five to ten minutes until some fat has rendered out and they are well coloured.
Use a slotted spoon to remove the bacon from the pan and set it aside.
Add the butter and oil to the pan, along with the potatoes, onion, leeks and some salt and pepper.Cook these over a medium heat for ten minutes, stirring frequently to stop them sticking to the pan. A modest amount of sticking is OK, these sticky bits will add to the flavour.
Add the stock and bring to the boil. Check the seasoning at this stage to decide whether you need more salt and pepper then adjust to your liking. Simmer for twenty five minutes then take the pan off the heat.