Peel the celery root and potatoes and cut into large pieces. Peel and quarter the onions. Fry the celery, potatoes and onions briefly in olive oil. Deglaze with half of the water. Add the vegetable stock and salt. Cook everything in about 20 minutes until soft. Then puree and fill the soup with the remaining approx. 350 ml of water. Season with salt and pepper and add the butter.
Cut the bread slices into cubes and finely chop the garlic. Fry the bread and garlic in olive oil until crispy and lightly salt.
Whip the cream until stiff and mix with the soup. This makes it nice and frothy.
Distribute on plates and top with the croutons as a topping.