Creamy Root Vegetable Soup

2 ounces butter or margarine
2 leeks or onions
4 medium sized carrots
3 turnips
4 potatoes
half a medium sized head of white cabbage
1 teaspoon salt, or to taste
4 quarts stock
1/3 cup minced chervil, chopped (optional)

Wash and peel vegetables. Slice them into small pieces.
Saute vegetables in butter and salt.
Turn off heat, cover pot, and let vegetables steam for 20 minutes.
Add stock and bring to boil.
Reduce heat, cover and allow to cook for 45 minutes, stirring occasionally.
Blend cooked soup in blender until it becomes creamy and even. Serve hot topped with croutons and chervil.