Put about 8 1/3 cups (2 L) of water to boil in a suitable container.
Add the beetroot, peeled and chopped. Boil until the vegetables soften.
To the boiled vegetables, add the butter and salt to taste. After about 5 min., remove the soup from the heat.
Blend it until it becomes homogeneous. Garnish with parsley. When serving, add a spoon of yoghurt if desired.