Method
Halve the prepared sweet potatoes lengthwise, then cut into thin slices. Put the coconut oil in a deep saucepan and place over a medium heat to melt, then add the onion and fry for 5 minutes, stirring, until starting to soften. Add the coriander and cumin seeds and stir to mix, then add the thinly sliced sweet potatoes and apple and cook for another 5-10 minutes, stirring regularly, until everything has softened and cooked down a little. Next, stir in the grated ginger, then pour in the coconut milk and the hot stock and mix well. Season to your taste with salt, then bring the pan up to a simmer and let it bubble gently for 5 minutes or so. When the potatoes and apples are soft enough to blend, take the pan off the heat and let it cool. Use a stick blender to blend the soup until smooth, or whizz it carefully in batches in a food processor. Return the soup to the pan to reheat gently before serving, with a small swirl of yoghurt and a scattering of chopped herbs on the top of each bowl.