Preheat your oven to 400°F (200°C). Cut the cauliflower into uniform florets. Toss the florets with 2 tablespoons of olive oil, salt, and pepper on a large baking sheet. Spread them in a single layer.

Roast for 25-30 minutes, or until the cauliflower is tender and golden brown. Stir halfway through.

While the cauliflower roasts, heat the remaining 1 tablespoon of olive oil in a large pot or Dutch oven over medium heat. Add the diced onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute.

Add the roasted cauliflower to the pot with the sautéed aromatics. Pour in the vegetable broth and milk. Bring to a simmer, then reduce heat to low and cook for 10-15 minutes.

Blend the soup until smooth using an immersion or standard blender.

Return the blended soup to low heat. Stir in Parmesan cheese (if using) and nutmeg. Adjust seasoning as needed. Warm through gently.

Ladle into bowls and garnish with chives or parsley. Serve immediately.