Melt the butter in a large pan, then fry the bacon until crisp.
Add onion, stirring until it starts to turn golden.
Tip in the leeks, celery and potatoes, stir well, then cover and turn down the heat. Cook gently for 5 mins, shaking the pan every now and then to make sure that the mixture doesn’t stick.
Pour in the stock, season well and bring to the boil.
Cover and simmer for 20 mins until the vegetables are soft.
Leave to cool for a few mins, then blend in a food processor in batches until smooth.
Return to the pan, pour in the cream and stir well. Taste and season if necessary.
Serve scattered with crispy bacon and cracked black pepper.