Fresh rosemary gives this leek and potato soup a lot of flavour. The truffle oil adds depth, but is not essential...except to me. I adore truffle oil. For extra truffle flavour, substitute truffle salt for the salt.
Melt butter in a large saucepan over a medium low heat. Add leeks, and cook and stir for 5 minutes then stir in rosemary, and celery. Cook until leeks and celery are soft, about 15 minutes.
Add potatoes, and enough water to just cover the potatoes. Simmer, stirring occasionally until potatoes are soft and cooked through. The broth should be starting to thicken. Scoop about 250ml into a potato ricer, squeeze potatoes through back into the soup. If you don't have a potato ricer, you can mash with a fork. Season to taste with salt and pepper.
Pour into bowls, sprinkle with more pepper and fresh parsley. If desired, add a few drops of truffle oil.