Melt butter in a large pot. Add garlic and onion and sauté until onions are tender. Add flour and stir until mixture clumps up.
Gradually whisk in milk. Add broth, salt, and cauliflower. Simmer for 15 minutes or until cauliflower is tender.
Transfer soup to a blender and pulse until smooth. Return soup to pot. Add cheeses and stir until melted and combined with soup. Serve with extra cheese if desired.
Notes
Flavor option: add 1-2 tablespoons adobo sauce from a can of chipotle peppers for a chipotle twist on this soup!