Mushroom Soup

8 to 12 ounces fresh mushrooms
1 small onion, sliced thin
1/4 cup butter or margarine
3 1/2 cups milk
1/3 cup flour, mixed to a smooth paste with
    1/4 cup cold milk 4 chicken bouillon cubes
Pinch of thyme
1/2 cup plain yogurt

 Brown the mushrooms and onions in butter for about 5 minutes.

Add milk, flour paste, chicken bouillon cubes and thyme. Heat over moderate heat, stirring frequently until thickened. Remove from heat. Season with salt and pepper. Stir a bit of the hot soup into the yogurt. Stir all of the yogurt mixture into the hot soup. Serve.