Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and chop the sweet potato into small chunks. Chop the cauliflower into small florets and chop the core into bite-size chunks. Set any larger leaves aside, but keep the tender, pale green leaves. Tumble the sweet potato, cauliflower and tender cauliflower leaves into a roasting tin and drizzle with 1 tbsp olive oil. Season. Slide into the oven and roast for 20 mins till the veg are browned all over and the leaves are crisp.
While the veg roast, peel and finely chop the onion. Peel and crush the garlic clove. Heat a pan over a medium heat for 2 mins. When warm, drizzle in 1/2 tbsp olive oil. Add the onion and turn the heat down to low and cook for 6-8 mins till softened. Add the garlic and 1 tsp turmeric and cook, stirring, for 1 min.
Add the roasted sweet potato and cauliflower to the pan (keep the cauliflower leaves for later). Toss together.
Crumble in the stock cube and pour in 700ml boiling water. Cover and simmer for 5 mins.
Using a stick blender, blitz till smooth – or you can ladle the soup into a blender and blitz to combine, then pour back into the pan to gently reheat. Taste and season the soup. Garnish with a few crispy cauliflower leaves.