Peel and dice the potatoes, peel the carrots and cut into small slices or dice as well. Peel and dice the sweet potatoes.
Put everything in a large saucepan and fill up with the water. Add the vegetable stock and the marjoram. Let simmer for about 20-30 minutes. The amount of water can vary, depending on how thin or thick you want the soup to be.
Puree the cashew nuts in a blender with the water to a very fine cream.
Add to the soup and bring to the boil briefly. Season to taste with pepper and salt. Blend briefly with a blender to give the soup a nice, creamy consistency.
Depending on the taste and tolerance, the vegetables can be supplemented with e.g. leeks, onions and garlic.