Place cauliflower florets in a single layer onto the prepared baking sheet. Add 2 tablespoons olive oil; season with salt and pepper, to taste. Gently toss to combine. Place garlic onto the baking sheet.

Place into oven and roast until cauliflower and garlic is tender and golden brown, about 30-35 minutes.

Heat remaining 1 tablespoon olive oil in a large stockpot or Dutch oven over medium heat. Add onion, and cook, stirring occasionally, until tender, about 3-5 minutes.

Stir in cauliflower and garlic. Bring to a boil; reduce heat and simmer, covered, until cauliflower is tender and falling apart, Puree with an immersion blender until desired consistency is reached.

Stir in heavy cream; season with salt and pepper, to taste.add more vegetable stock as needed until desired consistency is reached.

Serve immediately.