Prepare the Cauliflower: Rinse the cauliflower. Then cut the cauliflower in half, through the middle of the stem. Cut off the stem from both halves. Use your hands to break many of the florets away. Use a paring knife to cut bite-sized florets. Cut branch stems into small, bite-size pieces. You should have around 7 to 8 cups of florets and small stem pieces. Learn more about how to cut cauliflower.
Heat the oil in a large pot over medium heat. Add the onion, celery, and carrots. Sauté until the vegetables are tender, about 10 to 15 minutes. Add garlic and cook for about a minute longer. Then add water, bouillon, wine, nutritional yeast flakes, cashews, and chopped cauliflower florets and pieces. Bring to a boil, then reduce to a simmer. Simmer for 20 minutes until cauliflower is tender. Remove from heat and let cool slightly. Use tongs to remove about 3 cups of the cauliflower florets from the pot.
Blend the soup: Remove the center cap from your blender lid. Fill the blender only halfway. Secure the lid on the blender and top with a kitchen towel. Blend on low speed for a few seconds, then slowly increase speed to create a creamy soup. Then add some of the reserved florets and give it a few quick bursts to break up larger pieces into bits. Transfer to a bowl. Repeat with remaining ingredients from the pan. Again, add some of the remaining florets and pulse. Be sure to leave some florets as a topping.
Season with salt and pepper to taste. Serve with cauliflower florets, croutons, toasted nuts, and/or sriracha.