Line a cookie sheet with parchment paper or foil.
Place the cauliflower florets on the cookie sheet, add oil and salt and toss gently using your hands, just enough to coat the cauliflower with the oil.
Bake at 425 for 20-30 minutes or until the cauliflower edges turn brown.
While the cauliflower bakes, sauté the onions with the olive oil in a soup pot for 5-8 minutes until golden and translucent.
Add the garlic cloves and sauté for 1 minute more.
Add to the pot the roasted cauliflower (leaving 4-5 small florets for garnish), broth, salt, pepper, and nutmeg. Bring to a boil.
Lower the heat, cover, and simmer for 20-30 minutes.
Transfer in batches to a high-speed blender (best results) and add 2 tablespoons butter (or olive oil) and process until very smooth and light in color. Adjust seasoning as you like.
Garnish with a cauliflower floret and serve hot!