Using a dutch oven melt butter over medium heat. Add chopped onion. Cook until soft and translucent. Add garlic. Cook and stir for a minute more until fragrant.
Sprinkle flour over buttery onions. Cook and stir for about 1 minute. Gradually add milk and chicken broth. Stir to blend. Bring to a boil.
Add cauliflower and carrots. Reduce heat. Simmer uncovered for 12 - 15 minutes or until cauliflower is tender.
Ladle soup into individual oven safe bowls. Top soup with sliced Swiss cheese. Broil 4-6 in. from the heat for 3-5 minutes or until cheese is bubbly.