Preheat your oven to 425°F. Line a rimmed baking sheet with parchment paper (for easy cleanup).
Spread the cauliflower florets, chopped onion and cloves of garlic in an even layer on the rimmed baking sheet. Drizzle with olive oil, and season with kosher salt + black pepper.
Roast for about 45 minutes, until the cauliflower + onion is golden brown and caramelized.
Transfer the roasted veggies to your high speed blender, along with 2 cups of bone broth. Blend over medium high speed until smooth, about 60-90 seconds.
Pour the pureed cauliflower into your soup pot, over medium heat. Add in the heavy cream, then thin to your desired consistency with bone broth. I usually use 4 cups in total (including the 2 cups I already had in the blender). Allow warm through.
Serve warm, topped with chives (or other favorite fresh herbs) and black pepper.
Leftovers will keep in an airtight container refrigerated for at least 5 days, or frozen for at least 6 months.
See Notes below for more tips, FAQs and substitution options!