Peel and finely chop the onion and the sweet potatoes. Warm a pan for 1 min, then add 1 tbsp olive 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. Measure out 1 tsp of the coriander seeds and tip 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 of the cumin and the 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.
Meanwhile, tip the peanuts into the frying pan. Toast over a medium heat for 2-3 mins till the nuts are golden and smell nutty. Tip into a dry tea towel and cool for a few mins. Rub off the skins. Roughly chop.
When the veg are very soft, add half the peanuts 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. Top with the remaining peanuts.