Preheat the oven to 180C/170C fan/Gas 4. Peel and chop the sweet potato into small chunks. Chop the cauliflower into chunks, including the core. Add any of the tender leaves.Tumble on to a roasting tray and drizzle with 1 tbsp olive oil. Season. Slide into the oven and roast for 20 mins.
Peel and finely dice the onion. Peel and crush the garlic clove. Heat a pan. When warm, drizzle in 1/2 tbsp olive oil. Throw in the onion and cook on a low heat for 6-7 mins till softened. Add the garlic and cook for 1 min.
Carefully add the roasted sweet potato and cauliflower to the pan. Dust in the saffron powder. Toss together.
Pour in the water or homemade stock. Clamp on a lid and simmer for 5 mins.
Using a stick blender, blitz till smooth. Or ladle into the bowl of a food processor. Pour back into the pan to gently reheat. Taste and season the soup. Garnish with a few crispy cauliflower leaves.