Sweat the onion, garlic, celery and leek until well cooked;
Add potatoes, cover with stock and bring to a boil until potatoes are coming apart;
Add milk little by little and blend, checking the consistency as you go along (more milk if you want it thinner, less if you want it thicker) - you may leave some bigger chunks of potatoes if you want;
Add the bacon that was set aside, salt and black pepper to taste, I prefer not to add any salt as the bacon and stock take care of that, but plenty of freshly ground black pepper.