In a large saucepan, melt the Unsalted Butter (3 tablespoon).
Thinly slice the white and green parts of the Leek (2).
Mince the Yellow Onion (1).
Thinly slice the Celery (1 stalk).
Mince the Garlic (1 clove).
Peel and cube the Potato (1.5 pound). Place the cubes in a bowl with cold water, so they do not oxidize.
Add the leeks, celery, onion, garlic, Fresh Thyme (1 sprig), Fresh Sage (1 sprig) and a generous pinch of Salt (to taste). Cook over moderate heat, stirring occasionally, until the vegetables are soft and just starting to brown.
Add Chicken Stock (8 cup), Potato and Dried Tarragon (2 teaspoon). Bring to a boil, then simmer over moderately low heat until the vegetables are very tender, about 30 minutes.
Add Heavy Cream (1 cup) to the saucepan and simmer over moderate heat until slightly thickened, about 7 minutes. Season the soup with Salt (to taste) and Ground White Pepper (to taste); discard the thyme and sage sprigs.
At this point you can serve the soup as is, or blend with an immersion blender or blender. If you use a blender, make sure to blend in batches and only fill half full.
Cover the hole in the top with a towel, so as to let the steam release. Do not completely cover with the lid, or else you will have a huge mess on your hands. Blend on low.
Ladle the soup into bowls and serve! Feel free to garnish with any nice greens or herbs you might have around. I used celery leaves and drizzled with a bit of olive oil. Enjoy!