Sweet Potato Leek Soup

1 bunch leeks
2 Tbs butter
2 sweet potatoes
4 C water
1/2 cup milk
1/2 tsp thyme
1/4 tsp white pepper
1/2 tsp salt

Slice white and very light green part of leek. Rinse to get rid of any sand. Saute in butter until translucent and soft. Peel and thinly slice sweet potato. Add to leeks. Saute 3 minutes. Add water and thyme. Bring to boil. Simmer partly covered for 20 minutes until vegetables are very soft. Add milk, white pepper, and salt. Puree in blender. Return to pot and heat through.