Peel and finely chop the onion and the sweet potatoes. Warm a pan for 1 min, then add 1 tbsp oil and the onion and sweet potatoes. Season with salt and freshly ground pepper. Cover, turn the heat down to low and sweat for 10 mins till the veg have started to soften.
While the veg sweat, peel and grate the ginger. Tip 1 tsp of the coriander seeds into a dry frying pan. Toast over a medium heat for 2 mins till the seeds smell nutty and start to pop. Tip them into the veg. Add 1 tsp cumin and the grated ginger. Stir to mix.
Crumble the stock cube into the pan. Pour in 800ml boiling water (or just use hot homemade stock). Cover and turn the heat up. Bring to the boil, then turn the heat down a little and simmer for 10 mins till the veg are soft.
When the veg are very soft, add the peanut butter to the pan. Ladle the soup into a blender and blitz till smooth, or use a hand-held blender. Taste and season. Ladle into bowls and serve.