Heat oil in a six-quart pot on medium heat. Add onion and sauté until translucent, about 10 minutes. Add garlic cloves and sauté for another eight minutes, until golden.

Add cauliflower, zucchini, celery root, water, salt and pepper. Water should cover vegetables generously. Bring to a boil, then reduce heat, cover and simmer for one hour, stirring occasionally.

Blend soup using an immersion blender. If soup is too thick, add a little boiling water; if soup is too thin, simmer uncovered for a few minutes to thicken. Adjust seasoning to taste.