Heat a Dutch oven over medium heat. When the pot is hot, spray it with cooking spray. Add the garlic and onion. Sauté, stirring occasionally, until the garlic and onion have started to soften, about 4 minutes.

Add the milk and cauliflower. Season with salt and pepper to taste. Cover the pot and bring the soup to simmer. Simmer until the cauliflower is tender, about 25 minutes.

Stir 1 cup of cheddar cheese into the soup

Working in batches, transfer the soup to a blender and puree until smooth. Return the soup to the pot; reheat if necessary. Season with salt and pepper to taste.

Ladle the soup into bowls, top with the remaining cheddar and the scallions, and serve.