I use the same basic technique to make any soup. Start with some hard aromatic vegetables, cleaned and chopped, and some type of fat. You can use butter, oil or a combination of both. Onions, garlic, carrots, parsnips and celery root all work well. You can use a little bacon here too, but it's not necessary. Omit the carrots if you want a pale color or white soup. Sweat the vegetables in the fat until they are tender but not caramelized. Then add the primary ingredients like squash, beans, tomatoes or whatever. In this case it would be the leeks. Now sweat them some more until they are tender and add water, stock or a mixture of both and some peeled potatoes. Simmer the soup until the potatoes are tender. Throw in a handful of sea salt, some fresh black pepper and a few sprigs of thyme. Carefully puree the soup in a blender after letting it cool for a few minutes. Push it though a strainer if you want a refined mouth feel.
Using potatoes to thicken a pureed soup gives it a thick rich texture. This soup can easily be made vegan using olive oil and water and it is so rich that anyone would really like it. The taste of the leeks really comes through without bacon or chicken stock so give it a try for Spring!