In a large pot, melt the butter over medium heat. Add the chopped onion and sauté for about 5 minutes until translucent. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the cauliflower florets to the pot and stir to combine. Cook for about 5 minutes, allowing the cauliflower to soften slightly.
Pour in the vegetable broth and bring the mixture to a boil. Reduce the heat and let it simmer for 15-20 minutes, or until the cauliflower is tender.
Remove the pot from heat and use an immersion blender to puree the soup until smooth. Alternatively, you can carefully transfer the mixture to a blender in batches.
Return the pureed soup to the pot over low heat. Stir in the heavy cream, thyme, lemon juice, and season with salt and pepper to taste. Heat through but do not boil.
Serve hot, garnished with chopped chives or parsley.