Cut leek in rings and sauté ¼ of a leek with butter for a few minutes over medium heat. Set aside.
Meanwhile in a separate pan sauté the rest of the leek with the olive oil, parsley, celery, onion, garlic, and mushrooms until the vegetables are soft.
Add hot broth, season with salt and pepper and boil for 20 minutes.
Process 1 cup of soup in a blender until it is a smooth consistency. Add the pureed soup back in with the rest of the soup.
Turn on the heat and when the soup starts boiling stir in pasta, cook for the recommended cooking time on the packaging.
Let soup sit ten minutes before serving in individual bowls. Add sautéed leek rings, carrots, or greens for garnish.
