Melt butter in a large pot over medium heat.

Add onion and cook until softened, about 5 minutes.

Stir in flour and cook for 1 minute.

Gradually whisk in chicken broth until smooth.

Add cauliflower and bring to a boil. Reduce heat and simmer until cauliflower is tender, about 20 minutes.

Carefully blend the soup with an immersion blender or in a regular blender until smooth.

Stir in heavy cream, salt, and pepper. Heat through.

Serve hot.