Preheat oven to 425 degrees F.
Place chopped cauliflower and sliced onion on a large baking sheet. Toss with 2 tablespoons olive oil and season with salt and pepper. Place in preheated oven for 30-35 minutes, flipping the veggies halfway through.
Place butter in a soup pot over medium heat. Allow butter to melt before adding minced garlic. Cook the garlic for about 30 seconds or until fragrant.
Pour vegetable broth into soup pot and turn up the heat to medium-high.
Add the roasted cauliflower and onion to the soup pot. (Reserve roasted cauliflower if you wish to use for garnish). Season with nutmeg and a little salt and pepper. Bring the soup to a simmer and cook for about 15 minutes.
Turn off the heat and allow soup to cool slightly before transferring it to a blender. Be sure to work in batches if necessary and do not overfill the blender. If you have an immersion blender, you may use that instead. Pour the soup back into the soup pot if using a blender, and taste. Depending on whether your broth is salted or not, you may need to stir in a generous sprinkling of salt.
Serve warm garnished with chopped fresh parsley if desired.