Melt in a soup pot over medium heat:
Add and cook, stirring, until tender but not browned, about 10 minutes:
Stir in and cook for an additional 2 minutes:
Whisk in slowly:
Add:
Bring to a boil over high heat, then reduce to a simmer, partially cover, and cook, stirring occasionally, until the cauliflower or broccoli is very tender, about 15 minutes. Puree the soup with an immersion blender (or in batches in a food processor or regular blender) until smooth. Return to the pot and stir in:
Heat through, but do not boil. Taste and add:
Serve garnished with: