Carrots and leeks simmer together in chicken broth for a simple, nourishing soup. Fresh thyme gives the soup a delicate herbal note while sour cream, added just at the very end of cooking, delivers a lovely creaminess and pleasant acidic note.
Melt the ghee in the bottom of a Dutch oven or heavy stock pot over medium heat, and then drop in the leeks and thyme leaves. Sprinkle them with salt, stir, and then cover the pot. Allow the leeks to sweat in the hot fat about 5 minutes, or until they begin to turn tender.
Dump the celery and carrots into the pot, and then pour in the chicken broth. Increase the heat to medium-high. When the contents of the pot reach a boil, turn down the heat to medium, and simmer, covered, about 25 minutes or until the vegetables are fall-apart tender.
Turn off the heat, and then stir in the sour cream. Puree with an immersion blender until perfectly smooth, and then ladle into bowls to serve hot. Store any leftovers in a sealed container in the fridge for up to 5 days.