I almost always buy my cauliflower at the farmer's market and use it within a couple of days -- the freshness and quality of the cauliflower is the most important ingredient in this soup. If you can find yellow cauliflower (an heirloom variety) try it! The flavor is not very different from white cauliflower, but the end result is extremely pretty. The other extremely important factor in getting a great soup is the broth you use: there are many nasty, funky-flavored broths out there. I recommend you use Swanson's low-sodium organic chicken or vegetable broth, because the flavors are good without being too assertive for the mild flavor of the cauliflower.
Pour a couple of tablespoons of olive oil into your soup pot over medium heat. Add the diced onion and a pinch of salt and begin to sweat the onion (it should be turning translucent, not browning). Remove the stem from the cauliflower and break up the florets into small chunks. Toss the cauliflower into the pot with the onion and add just enough of the broth to not quite cover the cauliflower -- you probably won't use up the whole box. Place the lid on the pot and simmer for about 20-30 minutes, or until the cauliflower florets are softened enough to be easily broken apart with a spoon.
If you have a stick blender, you can just puree the soup in the pot (use the lid as a shield to prevent splatter). If not, ladle the soup into a blender or food processor and blend until very smooth.
I usually add about 1/3 cup of cream to my soup to get a nice consistency, but you can add regular milk if you want to cut down on calories. You can add more or less to get the consistency you desire -- I like my soup just viscous enough to float a drizzle of extra virgin olive oil on top. Add salt and pepper to taste.
To garnish the , grate a little parmesan on top, and drizzle with your favorite extra-virgin olive oil. Serve with toasted bread.