Preheat the oven to 400 degree F. Line a baking tray with parchment paper and keep it aside.
Cut the cauliflower base and separate florets; transfer them to the tray. Add the red onions to the same tray.
Toss the olive oil over the cauliflower along with the spices; salt, black pepper, cayenne pepper, and nutmeg. Coat them well.
Place it in the oven and roast for 30-35 min until the cauliflower is roasted, flipping it halfway through. Heat a heavy bottom pot over medium heat; add the roasted cauliflower along with the chicken broth. Let them simmer for 4-5 minutes.
Transfer them to a blender; puree the soup until smooth and creamy. Add the butter and blend again.
Pour the mixture into the same pot and simmer for 2-3 minutes. If the soup consistency is thick add some more broth to make it a little thin.
Spice up the soup with red chili flakes on top before serving and garnish with parsley, olive oil, and croutons on the side.