Creamy Roasted Cauliflower Soup

large head cauliflower (about 2 pounds), cut into bite-size florets

extra-virgin olive oil, divided

Fine sea salt

medium red onion, chopped

garlic, pressed or minced

vegetable broth

unsalted butter

fresh lemon juice, or more if needed

Scant ¼ teaspoon ground nutmeg

For garnish: 2 tablespoons finely chopped fresh flat-leaf parsley, chives and/or green onions