Melt butter in a large pot over medium heat. Add onion and garlic; cook and stir until onion is translucent, about 5 minutes.
STEP 3
Add potatoes and carrots; cook and stir until beginning to soften, about 5 minutes.
STEP 4
Pour in broth and bring to a boil. Stir in cauliflower, then cover, reduce heat, and simmer until vegetables are tender, 10 to 20 minutes. Remove from heat.
STEP 5
Purée soup with an immersion blender until smooth.
STEP 6
Return to the stove and stir in milk, salt, pepper, nutmeg, and sherry over low heat. Cook until heated through, 3 to 4 minutes.