Preheat oven to 400°F. Line a baking sheet with parchment paper.
In a large bowl, toss together the cauliflower florets, garlic, olive oil, oregano, salt and pepper until everything is evenly coated.
Spread the mixture out onto the prepared baking sheet and roast in the preheated oven for 25-30 minutes or until the cauliflower is golden brown and tender.
Transfer the roasted cauliflower to a large pot over medium heat. Pour in the vegetable broth and bring to a simmer. Simmer for 10 minutes.
Carefully transfer the soup to a blender (or use an immersion blender) and blend until smooth. Return it to the pot over low heat and stir in heavy cream if desired.
Season with additional salt and pepper if needed. Serve hot.