Begin by preparing your vegetables – peel and dice the potatoes, chop onion and garlic. Trim the leeks and discard the tough outer layer. Wash them thoroughly in cold running water, slice finely and drain.
In a large heavy based saucepan melt the butter over medium to low heat. Add the onions, garlic, leeks and potatoes and give them a good stir round, so they are well coated in butter. Season with salt and pepper, cover and let the vegetables sweat over low heat for about 15 minutes, or until the leeks are soft and tender.
Add the stock and milk and bring to a simmer. Be careful, when adding the milk to the soup, (better pour it slowly, stirring continuously) and, once you’ve actually added the milk, have the heat not too high, because it may boil over. Leave the soup covered and simmer gently for 15 minutes, until the potatoes are soft.
Meanwhile you can prepare the bacon – arrange the slices in a single layer over baking sheet (I cover it with parchment paper to save some greasy washing). Put them under the grill and set the oven to 160C/320F. Let them bake for 2 or 3 minutes – just until they get crispy. Turn once, if needed (I do it when slices are thicker). When the bacon is ready, leave to slightly cool, and break (or cut) into bytes.
When the vegetables are tender, take the soup off the heat and leave it to cool for few minutes. Transfer the soup into a blender, or puree it using a hand blender. Leave some small chunks, if you like it this way or make it smooth and velvety.
Check the seasoning and return the soup to the saucepan, to keep it hot till serving.
Serve hot, sprinkled with the bacon bytes. Or you omit the bacon, adding a swirl of crème fraîche and some freshly chopped parsley.
Your leek and potato soup with bacon is all set to enjoy!