Heat your oven to 220°C/Fan 200°C/Gas 7. Set a roasting dish on the top shelf to heat up.
Peel the sweet potatoes. Cut into 2-3cm cubes. Tumble into the warmed roasting dish. Drizzle over 1 tbsp oil. Season well. Roast for 20 mins. Halve, peel and finely chop the onion. Add it to the sweet potato. Roast for a further 10 mins, or till everything is tender and caramelised around the edges. Remove, but keep the oven on.
Whizz the sweet potato and onion with the coconut milk (save 2-3 tbsp for garnish), curry paste and 150ml water till smooth, adding more water if needed.
Pull the kale leaves from their woody stalks. Peel and finely chop or grate the garlic. Pile the kale and garlic into a bowl with 1 tbsp oil and a good pinch of salt. Massage the leaves till you can feel them soften a bit.
Arrange in a single layer on a large baking tray. Set on the top shelf of your oven. Roast for about 5 mins till crispy and golden around the edges. Serve alongside the sweet potato soup, which you can finish with a swirl of the reserved coconut milk.