Preheat your oven to 400°F.
Prepare a baking sheet with parchment paper.
Cut the cauliflower into small, even pieces and spread them on the baking sheet.
Drizzle 1 tablespoon of olive oil over the cauliflower and season with salt and pepper.
Roast in the oven for 25 minutes until golden brown.
In a large pot, melt half of the butter over medium-low heat.
Add the finely diced onion, carrot, and celery.
Sauté until the onions are translucent.
Add the roasted cauliflower and chicken broth to the pot.
Let it simmer for 20 minutes.
Use a blender to process the soup in batches until smooth.
Return the blended soup to the pot.
Stir in the heavy cream.
Season with salt and pepper to taste.