Put the remaining cauliflower, onion, celery root, turnip, celery, shallot, garlic, red pepper flakes, and 1/4 cup of the broth in a large pot over medium-low heat. Cook, stirring occasionally, for 10 minutes. Do not allow the bottom of the pot to brown. Stir in the remaining 4 3/4 cups of broth and the soymilk. Increase the heat to medium-high and bring to a simmer. Decrease the heat to medium and cook, stirring occasionally, for 15 minutes. Transfer to a blender and process on high speed until smooth. You may need to process the soup in batches depending on the size of your blender. Serve hot, garnished with the roasted cauliflower and nutmeg.