Melt in a large saucepan over medium heat:
Add and cook, stirring, until softened, 6 to 8 minutes:
Add:
Bring to a boil, then reduce to a simmer, cover, and cook until the cauliflower or broccoli is very tender, about 10 minutes. If desired, puree the soup with an immersion blender (or in batches in a food processor or regular blender) until smooth. Return to the pan and stir in:
Heat through, but do not boil. Season to taste with:
Garnish with, if desired: