Add the olive oil to a large pot over medium high heat, and saute the onion, carrots, and celery until they soften, about 8 minutes. Add in the garlic, thyme, and red pepper flakes and stir briefly, about 1 more minute.
Pour in the water, then add in the cauliflower, salt, and pepper. Bring the liquid to a boil over high heat, then once the soup is boiling, lower the heat to a gentle simmer and cover the pot with a lid. Let the soup cook until the cauliflower is very tender, about 20 minutes.
Transfer the soup to a high-speed blender, and carefully blend until it looks creamy, about 60 seconds. Be sure to remove the vent in your blender's lid and cover it with a thin dish towel, to allow steam to safely escape without splattering. (Otherwise the steam pressure could blow the lid off!) Alternatively, you can use an immersion blender to blend the soup directly in the pot.
Once the soup is blended, return it to the pot over medium heat and stir in the shredded cheese. Stir until the cheese has melted, then adjust any seasoning to taste. (You may need to add an extra 1/4 teaspoon of salt, or more depending on how much flavor the cheese added.) Serve warm, with extra shredded cheese or fresh thyme on top. Leftovers can be stored in an airtight container in the fridge for up to 1 week.
Notes
Nutrition information is for 1 of 6 servings, which is about 1.3 cups of soup. This information is automatically calculated, and is just an estimate, not a guarantee.• If you have a head of cauliflower that is 1.5 pounds, you can roast the remaining cauliflower as a topping for this soup. To roast the cauliflower, preheat the oven to 400ºF and toss the cauliflower in a rimmed baking sheet with a drizzle of olive oil. Arrange in a single layer, sprinkle with salt, and roast until the cauliflower is tender, about 20 minutes.• This recipe calls for water instead of broth, which is why it needs a generous seasoning of salt. I use Real Salt brand when testing my recipes, so you may need to use more or less salt if you're using a different variety. You can always start with less and add more to taste, but if the soup tastes bland, please don't be afraid to add more salt! If you decide to start with a store-bought veggie or chicken broth, be sure to cut-back on the salt, since the broth will already contain sodium.