Preheat oven to 425º Fahrenheit. Lightly oil a baking sheet or coat with nonstick spray.
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 whole head of garlic on individual flat piece of foil paper, sprinkle garlic, drizzle with 1 tablespoon olive oil and wrap tightly. Place in the same baking sheet with the cauliflower and transfer pan to the preheated oven and roast until cauliflower and garlic are both soft and caramelized, about 40 - 45 minutes. Remove from oven and allow to cool. Let garlic cool before squeezing cloves from skin.
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 3-4 minutes. Stir in stock, bring to a boil; reduce heat and simmer, covered, for 5 minutes. Add in cauliflower and roasted garlic. Bring to a boil; reduce heat and simmer, cover and cook an additional 10 minutes. Puree with an immersion blender (or regular blender- in batches) until desired consistency is reached. Stir in softened butter, squeeze fresh lemon juice and season with salt and pepper (if needed). Top with garnish of choice.
Serve immediately. Enjoy!
** For a thinner soup, add more stock as needed until desired consistency is reached.