Preheat the oven to 425 degrees Fahrenheit. If desired, line a large, rimmed baking sheet with parchment paper for easy cleanup.

On the baking sheet, toss the cauliflower with 2 tablespoons of the olive oil until lightly and evenly coated in oil. Arrange the cauliflower in a single layer and sprinkle lightly with salt. Bake until the cauliflower is tender and caramelized on the edges, 25 to 35 minutes, tossing halfway.

In a Dutch oven or soup pot, warm the remaining 1 tablespoon olive oil over medium heat until shimmering. Add the onion and ¼ teaspoon salt. Cook, stirring occasionally, until the onion is softened and turning translucent, 5 to 7 minutes.

Add the garlic and cook, stirring constantly, until fragrant, about 30 seconds, then add the broth.

Reserve 4 of the prettiest roasted cauliflower florets for garnish. Then transfer the remaining cauliflower to the pot. Increase the heat to medium-high and bring the mixture to a simmer, then reduce the heat as necessary to maintain a gentle simmer. Cook, stirring occasionally, for 20 minutes.

Remove the pot from the heat and let it cool for a few minutes. Transfer the hot soup to a blender, working in batches if necessary.

Add the butter and blend until smooth. Add the lemon juice and nutmeg and blend again.

Add additional salt, to taste. Blend again.

Top individual bowls of soup with 1 roasted cauliflower floret and a sprinkle of chopped parsley, green onion and/or chives.