Heat oil in a stock pot over medium-high heat, and cook onion for about 5 minutes.
Add carrot, celery and garlic and sauté for another 5 minutes.
Add the cauliflower and potato.
Add the vegetable stock, salt and pepper and bring to a boil. Reduce heat, cover, and simmer until all vegetables are very tender, for about 15 minutes.
Add milk and bring to boil.
Puree soup with a stick blender or food processor until smooth and strain it through a sieve.
Pour the hot soup into warmed bowls and garnish as desired with cheese, chili flakes, red chili pepper, chopped chives or parsley. Serve with cheese toast bread or bread croutons.