Heat the olive oil in a large pot over medium heat. After 90 seconds when the oil is hot, add the onion and celery and sauté for 7-8 minutes, until the vegetables are tender.
Add the garlic, chopped rosemary, chopped parsley, sage, thyme, and black pepper. Cook for 3 more minutes, stirring from time to time.
Add the nutritional yeast, mustard, cauliflower, white beans, and vegetable stock. Bring to a boil over high heat, then reduce to a simmer over medium-low heat. Cover and cook for 20 minutes, or until the cauliflower is completely tender.
Remove the pot from the heat. Using an immersion blender, blend the soup until smooth. Alternatively, carefully transfer the soup to a blender in batches and blend until smooth.
Return the soup to the pot and stir in the truffle oil. Season with salt to taste. If the soup is too thick, add additional vegetable stock or water until it reaches your preferred thickness.
Ladle the soup into bowls and drizzle with truffle oil. Garnish with the rosemary leaves and coarse ground black pepper.