Preheat your oven to 425°F. For easy cleanup, line a large rimmed baking sheet with parchment paper if you like.

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

When the cauliflower is almost done, warm the remaining 1 tablespoon olive oil in a Dutch oven or soup pot over medium heat until shimmering. Add the chopped onion and 1/4 teaspoon salt. Cook, stirring occasionally, for 5 to 7 minutes, until the onion is softened and translucent.

Add the garlic and cook, stirring constantly, for about 30 seconds until fragrant. Then pour in the vegetable broth.

Set aside 4 of the prettiest roasted cauliflower florets for garnish. Add the rest of the cauliflower to the pot. Turn the heat to medium-high and bring to a simmer, then lower the heat to maintain a gentle simmer. Cook, stirring occasionally, for 20 minutes to let the flavors meld.

Remove the pot from the heat and let it cool for a few minutes. Carefully transfer the hot soup to a blender in batches (do not fill past the maximum fill line).

Add the butter and blend until smooth. Add the lemon juice and nutmeg, then blend again. Taste and add more salt as needed (usually 1/4 to 3/4 teaspoon more, depending on the broth). If it needs more brightness, add a little extra lemon juice and blend once more.

Ladle the soup into bowls. Garnish each with one reserved roasted cauliflower floret and a sprinkle of chopped parsley, green onion, or chives. Store leftovers in a covered container in the fridge for up to 4 days, or freeze for several months.