Heat olive oil in a large pot over medium heat. Add the chopped onion and sauté for 5 minutes until softened.
Add garlic and cook for 1 minute, then add the cauliflower florets, broth, thyme, red pepper flakes, salt, and pepper. Bring to a boil.
Reduce heat to medium-low and simmer for 15-20 minutes until the cauliflower is very tender and easily pierced with a fork.
Use an immersion blender to puree the soup until smooth and creamy, or carefully transfer to a blender in batches.
Stir in the massaged kale and nutritional yeast if using. Cook for 3-5 minutes until the kale is tender. Serve hot.