Prepare the onion and garlic. Fry the garlic and onion in butter until translucent, about 5-7 minutes, and place in a saucepan.
Prepare the remaining vegetables. Add the celery root, carrots and potatoes to the pan with the onion and garlic.
Prepare the soup base. Fill the pan with hot water, the liquid should completely cover the vegetables. When the water boils, reduce the heat to minimum and cook the vegetables for 30 minutes.
Make a puree soup. When all the ingredients in the soup are ready, puree them using a blender. Pour the cream into the pan, add salt and pepper. Return the pan to the heat and bring to a boil.