Peel and thinly slice sweet potatoes and carrots. Finely chop the onion and celery. Place all in a large soup pot.
Add the vegetable stock. The stock should just barely cover all of the vegetables.
Add sage, rosemary, and thyme as well as salt and pepper to taste. Bring to the boil and cook until the vegetables are soft, approximately 20-30 minutes.
Let cool for about 10 minutes. Remove and reserve 1/2-1 cup of the liquid. Blend the rest of the soup in batches (may need to add a little of the reserved stock to get it smooth) and return to the pot. It should be a thick, smooth paste.
Over medium heat, add the milk and sour cream and blend in well. Depending on preference as to thickness, the amounts may be adjusted or some of the reserved stock added back.
Check the flavor and add more herbs, salt, or pepper as needed.
Once reheated, serve and enjoy.
Note: very good served with a dollop of sour cream and a dusting of sage and thyme. Can be frozen.
There are 199 calories in 1 serving of Sweet Potato & Carrot Soup.