Heat 2 tablespoons of the olive oil in a large pot over medium heat. Add chopped onion, chopped leeks, and sliced mushrooms and cook for about 4-5 minutes. Add minced garlic and cook for another couple of minutes.
Now add white sweet potatoes and water, stir well and bring the mixture to a boil. Reduce the heat to low and simmer for 20-25 minutes until white sweet potatoes are tender. Season with salt and cardamom.
When the soup is done, add cashew cream and use the immersion blender to puree the soup completely (or you can transfer the mixture to a blender and blend until smooth and creamy).
Put the soup back to the stove and simmer for another couple of minutes on low heat.
Feel free to add more water if the soup is too thick to your liking and adjust seasonings accordingly.
Pour the soup into bowls, garnish with roasted mushrooms, thyme, white and black sesame seeds...