Place cashews in a cup or small bowl and add just enough boiling water to cover. Let soak 15-30 minutes (or overnight).
Heat oil in a medium soup pot and add diced onion and shallots. Saute until onions and shallots are translucent and beginning to brown (about 8 minutes).
Add mushrooms and saute for 3 more minutes.
Add cauliflower, vegetable broth, and herbs. Bring to a boil, then reduce heat, cover, and simmer for 15 minutes.
While soup is simmering, drain the cashews, reserving their liquid. Place cashews in a blender with about 2 tablespoons of the soaking liquid. Blend until very smooth, adding more soaking liquid as required to puree the mixture.
Stir cashew cream into the soup.
Transfer soup into the blender and blend until smooth (or use an immersion blender directly in the soup pot). Serve hot, garnished with chopped scallions if desired.