Add oil and butter to pan and heat. Add leeks and cook on low for 10 minutes.
Add cauliflower, potatoes and stock. Stir. Season with salt,pepper, garlic.
Cover and bring to a boil. Then simmer for about 20 min or until cauliflower and potatoes are soft. Cool slightly.
Puree in a blender or food processor until smooth. Careful! You might have to do that in batches. Return to pan and heat. Check for seasoning. You might have to add some chicken stock or milk to adjust consistency.
Pour into soup bowls and garnish with chopped parsley.